[DOCS] Disable shard allocation - data nodes only (#72265)

When doing a rolling restart we recommend disabling shard allocation to
avoid unnecessary recoveries. However, this advise is unnecessary or
even harmful when restarting nodes that do not carry any data like a
pure ML node.

docs/reference/setup/restart-cluster.asciidoc

@@ -209,7 +209,7 @@ GET _cat/nodes
 . *Reenable shard allocation.*
 +
 --
-Once the node has joined the cluster, remove the
+For data nodes, once the node has joined the cluster, remove the
 `cluster.routing.allocation.enable` setting to enable shard allocation and start
 using the node:
 

docs/reference/upgrade/disable-shard-alloc.asciidoc

@@ -1,11 +1,11 @@
 
-When you shut down a node, the allocation process waits for
+When you shut down a data node, the allocation process waits for
 `index.unassigned.node_left.delayed_timeout` (by default, one minute) before
 starting to replicate the shards on that node to other nodes in the cluster,
 which can involve a lot of I/O. Since the node is shortly going to be
-restarted, this I/O is unnecessary. You can avoid racing the clock by 
+restarted, this I/O is unnecessary. You can avoid racing the clock by
 <<cluster-routing-allocation-enable,disabling allocation>> of replicas before
-shutting down the node:
+shutting down <<data-node,data nodes>>:
 
 [source,console]
 --------------------------------------------------

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -103,7 +103,7 @@ a node.
 . If you use {es} {security-features} to define realms, verify that your realm
 settings are up-to-date. The format of realm settings changed in version 7.0, in
 particular, the placement of the realm type changed. See
-<<realm-settings,Realm settings>>. 
+<<realm-settings,Realm settings>>.
 
 . *Start the upgraded node.*
 +
@@ -122,8 +122,9 @@ GET _cat/nodes
 +
 --
 
-Once the node has joined the cluster, remove the `cluster.routing.allocation.enable`
-setting to enable shard allocation and start using the node:
+For data nodes, once the node has joined the cluster, remove the
+`cluster.routing.allocation.enable` setting to enable shard allocation and start
+using the node:
 
 [source,console]
 --------------------------------------------------