Minor update to interal distributed guide for multi-project (#125556)

This PR adds a minimal amount information about multi-project so that
the existing distributed guide is up-to-date.

Relates: https://github.com/elastic/elasticsearch/pull/123565 Resolves:
ES-10985

docs/internal/DistributedArchitectureGuide.md

@@ -110,6 +110,16 @@ to communicate with Elasticsearch.
 
 ### Cluster State
 
+[ClusterState]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/cluster/ClusterState.java
+[Metadata]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java
+[ProjectMetadata]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java
+
+The [Metadata] of a [ClusterState] is persisted on disk and comprises information from two categories:
+1. Cluster scope information such as `clusterUUID`, `CoordinationMetadata`
+2. Project scope information ([ProjectMetadata]) such as indices and templates belong to each project.
+
+Some concepts are applicable to both cluster and project scopes, e.g. [persistent tasks](#persistent-tasks). The state of a persistent task is therefore stored accordingly depending on the task's scope.
+
 #### Master Service
 
 #### Cluster State Publication
@@ -306,7 +316,7 @@ policies.
 
 ### How cluster capacity is determined
 
-[AutoscalingMetadata][] implements [Metadata.Custom][] in order to persist autoscaling policies. Each
+[AutoscalingMetadata][] implements [Metadata.ClusterCustom][] in order to persist autoscaling policies. Each
 Decider is an implementation of [AutoscalingDeciderService][]. The [AutoscalingCalculateCapacityService][]
 is responsible for running the calculation.
 
@@ -322,7 +332,7 @@ calls [through the CapacityResponseCache][], into the `AutoscalingCalculateCapac
 concurrent callers.
 
 [AutoscalingMetadata]: https://github.com/elastic/elasticsearch/blob/v8.13.2/x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/AutoscalingMetadata.java#L38
-[Metadata.Custom]: https://github.com/elastic/elasticsearch/blob/v8.13.2/server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java#L141-L145
+[Metadata.ClusterCustom]: https://github.com/elastic/elasticsearch/blob/f461731a30a6fe55d7d7b343d38426ddca1ac873/server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java#L147
 [AutoscalingDeciderService]: https://github.com/elastic/elasticsearch/blob/v8.13.2/x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/capacity/AutoscalingDeciderService.java#L16-L19
 [AutoscalingCalculateCapacityService]: https://github.com/elastic/elasticsearch/blob/v8.13.2/x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/capacity/AutoscalingCalculateCapacityService.java#L43
 
@@ -505,6 +515,8 @@ The [Task management API] also exposes an endpoint where a task ID can be specif
 
 [PersistentTaskPlugin]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/plugins/PersistentTaskPlugin.java
 [PersistentTasksExecutor]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/PersistentTasksExecutor.java
+[PersistentTasksExecutor.Scope.Cluster]:https://github.com/elastic/elasticsearch/blob/f461731a30a6fe55d7d7b343d38426ddca1ac873/server/src/main/java/org/elasticsearch/persistent/PersistentTasksExecutor.java#L52
+[PersistentTasksExecutor.Scope.Project]:https://github.com/elastic/elasticsearch/blob/f461731a30a6fe55d7d7b343d38426ddca1ac873/server/src/main/java/org/elasticsearch/persistent/PersistentTasksExecutor.java#L48
 [PersistentTasksExecutorRegistry]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/PersistentTasksExecutorRegistry.java
 [PersistentTasksNodeService]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/PersistentTasksNodeService.java
 [PersistentTasksClusterService]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/PersistentTasksClusterService.java
@@ -513,13 +525,14 @@ The [Task management API] also exposes an endpoint where a task ID can be specif
 [HealthNodeTaskExecutor]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/health/node/selection/HealthNodeTaskExecutor.java
 [SystemIndexMigrationExecutor]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/upgrades/SystemIndexMigrationExecutor.java
 [PersistentTasksCustomMetadata]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/PersistentTasksCustomMetadata.java
+[ClusterPersistentTasksCustomMetadata]:https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/persistent/ClusterPersistentTasksCustomMetadata.java
 [PersistentTasksCustomMetadata.PersistentTask]:https://github.com/elastic/elasticsearch/blob/d466ad1c3c4cedc7d5f6ab5794abe7bfd72aef4e/server/src/main/java/org/elasticsearch/persistent/PersistentTasksCustomMetadata.java#L305
 
 Up until now we have discussed only ephemeral tasks. If we want a task to survive node failures, it needs to be registered as a persistent task at the cluster level.
 
-Plugins can register persistent tasks definitions by implementing [PersistentTaskPlugin] and returning one or more [PersistentTasksExecutor] instances. These are collated into a [PersistentTasksExecutorRegistry] which is provided to [PersistentTasksNodeService] active on each node in the cluster, and a [PersistentTasksClusterService] active on the master.
+Plugins can register persistent tasks definitions by implementing [PersistentTaskPlugin] and returning one or more [PersistentTasksExecutor] instances. These are collated into a [PersistentTasksExecutorRegistry] which is provided to [PersistentTasksNodeService] active on each node in the cluster, and a [PersistentTasksClusterService] active on the master. A [PersistentTasksExecutor] can declare either [project][PersistentTasksExecutor.Scope.Project] or [cluster][PersistentTasksExecutor.Scope.Cluster] scope, but not both. A project scope task is not able to access data on a different project.
 
-The [PersistentTasksClusterService] runs on the master to manage the set of running persistent tasks. It periodically checks that all persistent tasks are assigned to live nodes and handles the creation, completion, removal and updates-to-the-state of persistent task instances in the cluster state (see [PersistentTasksCustomMetadata]).
+The [PersistentTasksClusterService] runs on the master to manage the set of running persistent tasks. It periodically checks that all persistent tasks are assigned to live nodes and handles the creation, completion, removal and updates-to-the-state of persistent task instances in the cluster state (see [PersistentTasksCustomMetadata] and [ClusterPersistentTasksCustomMetadata]).
 
 The [PersistentTasksNodeService] monitors the cluster state to:
  - Start any tasks allocated to it (tracked in the local [TaskManager] by an [AllocatedPersistentTask])
@@ -529,7 +542,7 @@ If a node leaves the cluster while it has a persistent task allocated to it, the
 
 Some examples of the use of persistent tasks include:
  - [ShardFollowTasksExecutor]: Defined by [cross-cluster replication](#cross-cluster-replication-ccr) to poll a remote cluster for updates
- - [HealthNodeTaskExecutor]: Used to schedule work related to monitoring cluster health
+ - [HealthNodeTaskExecutor]: Used to schedule work related to monitoring cluster health. This is currently the only example of a cluster scope persistent task.
  - [SystemIndexMigrationExecutor]: Manages the migration of system indices after an upgrade
 
 ### Integration with APM