[DOCS] Note required node roles and data tiers (#74566)

Closes #74528 and #74565

docs/reference/datatiers.asciidoc

@@ -44,7 +44,8 @@ While they are also responsible for indexing, content data is generally not inge
 as time series data such as logs and metrics. From a resiliency perspective the indices in this
 tier should be configured to use one or more replicas.
 
-New indices are automatically allocated to the <<content-tier>> unless they are part of a data stream.
+The content tier is required. System indices and other indices that aren't part
+of a data stream are automatically allocated to the content tier.
 
 [discrete]
 [[hot-tier]]
@@ -56,8 +57,8 @@ Nodes in the hot tier need to be fast for both reads and writes,
 which requires more hardware resources and faster storage (SSDs).
 For resiliency, indices in the hot tier should be configured to use one or more replicas.
 
-New indices that are part of a <<data-streams, data stream>> are automatically allocated to the
-hot tier.
+The hot tier is required. New indices that are part of a <<data-streams,
+data stream>> are automatically allocated to the hot tier.
 
 [discrete]
 [[warm-tier]]

docs/reference/how-to/use-elasticsearch-for-time-series-data.asciidoc

@@ -13,10 +13,16 @@ using {kib} and other {stack} features.
 tiers>> to automatically move older data to nodes with less expensive hardware
 as it ages. This helps improve performance and reduce storage costs.
 
-The hot tier is required. The warm, cold, and frozen tiers are optional. Use
-high-performance nodes in the hot and warm tiers for faster indexing and faster
-searches on your most recent data. Use slower, less expensive nodes in the cold
-and frozen tiers to reduce costs.
+The hot and content tiers are required. The warm, cold, and frozen tiers are
+optional.
+
+Use high-performance nodes in the hot and warm tiers for faster
+indexing and faster searches on your most recent data. Use slower, less
+expensive nodes in the cold and frozen tiers to reduce costs.
+
+The content tier is not typically used for time series data. However, it's
+required to create system indices and other indices that aren't part of a data
+stream.
 
 The steps for setting up data tiers vary based on your deployment type:
 

docs/reference/modules/node.asciidoc

@@ -36,7 +36,18 @@ set `node.roles`, the node is assigned the following roles:
 [IMPORTANT]
 ====
 If you set `node.roles`, ensure you specify every node role your cluster needs.
-Some {stack} features require specific node roles:
+Every cluster requires the following node roles:
+
+* `master`
+* {blank}
++
+--
+`data_content` and `data_hot` +
+OR +
+`data`
+--
+
+Some {stack} features also require specific node roles:
 
 - {ccs-cap} and {ccr} require the `remote_cluster_client` role.
 - {stack-monitor-app} and ingest pipelines require the `ingest` role.
@@ -223,8 +234,9 @@ assign data nodes to specific tiers: `data_content`,`data_hot`, `data_warm`,
 `data_cold`, or `data_frozen`. A node can belong to multiple tiers, but a node
 that has one of the specialized data roles cannot have the generic `data` role.
 
+[role="xpack"]
 [[data-content-node]]
-==== [x-pack]#Content data node#
+==== Content data node
 
 Content data nodes accommodate user-created content. They enable operations like CRUD,
 search and aggregations.
@@ -235,8 +247,9 @@ To create a dedicated content node, set:
 node.roles: [ data_content ]
 ----
 
+[role="xpack"]
 [[data-hot-node]]
-==== [x-pack]#Hot data node#
+==== Hot data node
 
 Hot data nodes store time series data as it enters {es}. The hot tier must be fast for
 both reads and writes, and requires more hardware resources (such as SSD drives).
@@ -247,8 +260,9 @@ To create a dedicated hot node, set:
 node.roles: [ data_hot ]
 ----
 
+[role="xpack"]
 [[data-warm-node]]
-==== [x-pack]#Warm data node#
+==== Warm data node
 
 Warm data nodes store indices that are no longer being regularly updated, but are still being
 queried. Query volume is usually at a lower frequency than it was while the index was in the hot tier.
@@ -260,8 +274,9 @@ To create a dedicated warm node, set:
 node.roles: [ data_warm ]
 ----
 
+[role="xpack"]
 [[data-cold-node]]
-==== [x-pack]#Cold data node#
+==== Cold data node
 
 Cold data nodes store read-only indices that are accessed less frequently. This tier uses less performant hardware and may leverage searchable snapshot indices to minimize the resources required.
 
@@ -271,8 +286,9 @@ To create a dedicated cold node, set:
 node.roles: [ data_cold ]
 ----
 
+[role="xpack"]
 [[data-frozen-node]]
-==== [x-pack]#Frozen data node#
+==== Frozen data node
 
 The frozen tier stores <<partially-mounted,partially mounted indices>>
 exclusively. We recommend you use dedicated nodes in the frozen tier.

docs/reference/tab-widgets/data-tiers.asciidoc

@@ -23,6 +23,9 @@ a <<restart-cluster-rolling,rolling restart>>.
 
 [source,yaml]
 ----
+# Content tier
+node.roles: [ data_content ]
+
 # Hot tier
 node.roles: [ data_hot ]
 
@@ -41,7 +44,7 @@ assign other nodes to more than one tier.
 
 [source,yaml]
 ----
-node.roles: [ data_hot, data_warm ]
+node.roles: [ data_content, data_hot, data_warm ]
 ----
 
 Assign your nodes any other roles needed for your cluster. For example, a small