[DOCS] Add info about ILM and unallocated shards. (#56655)

* [DOCS] Add info about ILM and unallocated shards.

* Incorporated review feedback.

* Update docs/reference/ilm/actions/ilm-allocate.asciidoc

Co-authored-by: James Rodewig <james.rodewig@elastic.co>

* Apply suggestions from code review

Co-authored-by: James Rodewig <james.rodewig@elastic.co>

* Fix xref

Co-authored-by: James Rodewig <james.rodewig@elastic.co>

docs/reference/ilm/actions/ilm-allocate.asciidoc

@@ -7,16 +7,16 @@ Phases allowed: warm, cold.
 Updates the index settings to change which nodes are allowed to host the index shards
 and change the number of replicas.
 
+The allocate action is not allowed in the hot phase. 
+The initial allocation for the index must be done manually or via 
+<<indices-templates, index templates>>.
+
 You can configure this action to modify both the allocation rules and number of replicas, 
 only the allocation rules, or only the number of replicas. 
-
 For more information about how {es} uses replicas for scaling, see
 <<scalability>>. See <<shard-allocation-filtering>> for more information about
 controlling where {es} allocates shards of a particular index.
 
-NOTE: The allocate action is not allowed in the hot phase. 
-The initial allocation for the index must be done manually or via index templates.
-{ilm-init} doesn't handle index allocation during the hot phase.
 
 [[ilm-allocate-options]]
 ==== Options

docs/reference/ilm/ilm-index-lifecycle.asciidoc

@@ -36,6 +36,15 @@ the index must be older than the minimum age of the next phase.
 The minimum age defaults to zero, which causes {ilm-init} to move indices to the next phase
 as soon as all actions in the current phase complete. 
 
+If an index has unallocated shards and the <<cluster-health,cluster health status>> is yellow, 
+the index can still transition to the next phase according to its {ilm} policy.
+However, because {es} can only perform certain clean up tasks on a green
+cluster, there might be unexpected side effects. 
+
+To avoid increased disk usage and reliability issues,
+address any cluster health problems in a timely fashion.
+
+
 [discrete]
 [[ilm-phase-execution]]
 === Phase execution