Recommend index templates when using ILM Rollover (elastic#35922)

Because rollover does not propogate the ILM policy in use, add a note to
the docs recommending the use of index templates when using rollover.

docs/reference/ilm/set-up-lifecycle-policy.asciidoc

@@ -45,6 +45,7 @@ To set the policy for an index there are two options:
 1. Apply the policy to an index template and bootstrap creating the first index
 2. Apply the policy to a new index in a create index request
 
+[[applying-policy-to-template]]
 === Applying a policy to an index template
 
 beta[]

docs/reference/ilm/using-policies-rollover.asciidoc

@@ -21,6 +21,13 @@ met. Because the criteria are checked periodically, the index might grow
 slightly beyond the specified threshold. To control how often the critera are
 checked, specify the `indices.lifecycle.poll_interval` cluster setting.
 
+IMPORTANT: New indices created via rollover will not automatically inherit the
+policy used by the old index, and will not use any policy by default. Therefore,
+it is highly recommended to apply the policy via
+<<applying-policy-to-template,index template>>, including a Rollover alias
+setting, for your indices which specifies the policy you wish to use for each
+new index.
+
 The rollover action takes the following parameters:
 
 .`rollover` Action Parameters