Clarify S3 repository storage class parameter (#35400)

Today it is unclear that the `storage_class` parameter to an S3 repository only
affects new objects and does not rewrite any existing objects. This commit
clarifies this point.

docs/plugins/repository-s3.asciidoc

@@ -208,8 +208,12 @@ The following settings are supported:
 
 `storage_class`::
 
-    Sets the S3 storage class type for the backup files. Values may be
-    `standard`, `reduced_redundancy`, `standard_ia`. Defaults to `standard`.
+    Sets the S3 storage class for objects stored in the snapshot repository.
+    Values may be `standard`, `reduced_redundancy`, `standard_ia`.
+    Defaults to `standard`. Changing this setting on an existing repository
+    only affects the storage class for newly created objects, resulting in a
+    mixed usage of storage classes. Additionally, S3 Lifecycle Policies can
+    be used to manage the storage class of existing objects.
     Due to the extra complexity with the Glacier class lifecycle, it is not
     currently supported by the plugin. For more information about the
     different classes, see http://docs.aws.amazon.com/AmazonS3/latest/dev/storage-class-intro.html[AWS Storage Classes Guide]