Add note to CCR docs about mapping/alias updates

This commit adds a note to the docs clarifying that it is not possible
to manually update the mapping nor the aliases of a follower index.

docs/reference/ccr/overview.asciidoc

@@ -112,7 +112,8 @@ governed by settings that you can configure when you create the follower index
 in {kib} or by using the {ref}/ccr-put-follow.html[create follower API].
 
 Mapping updates applied to the leader index are automatically retrieved
-as-needed by the follower index.
+as-needed by the follower index. It is not possible to manually modify the
+mapping of a follower index.
 
 Settings updates applied to the leader index that are needed by the follower
 index are automatically retried as-needed by the follower index. Not all
@@ -120,7 +121,8 @@ settings updates are needed by the follower index. For example, changing the
 number of replicas on the leader index is not replicated by the follower index.
 
 Alias updates applied to the leader index are automatically retrieved by the
-follower index.
+follower index. It is not possible to manually modify an alias of a follower
+index.
 
 NOTE: If you apply a non-dynamic settings change to the leader index that is
 needed by the follower index, the follower index will go through a cycle of