vdo: document versions that have read cache options (#53291)

The "readcache" and "readcachesize" options, which correspond to
the same options in the "vdo create" and "vdo modify" commands,
have been removed in VDO version 6.2.

Signed-off-by: Bryan Gurney <bgurney@redhat.com>

lib/ansible/modules/system/vdo.py

@@ -127,6 +127,7 @@ options:
               volumes will maintain their previously configured
               setting unless a different value is specified in the
               playbook.
+            - The read cache feature is available in VDO 6.1 and older.
         type: str
         choices: [ disabled, enabled ]
     readcachesize:
@@ -142,6 +143,7 @@ options:
               Existing volumes will maintain their previously
               configured setting unless a different value is specified
               in the playbook.
+            - The read cache feature is available in VDO 6.1 and older.
         type: str
     emulate512:
         description: