Apply suggestions from code review

Co-Authored-By: Matthew Hodgson <matthew@arasphere.net>

api/client-server/key_backup.yaml

@@ -67,7 +67,7 @@ paths:
       responses:
         200:
           description:
-            The version of the new backup.
+            The version id of the new backup.
           schema:
             type: object
             properties:
@@ -123,7 +123,7 @@ paths:
                     }
                   }
               count:
-                description: The number of key stored in the backup.
+                description: The number of keys stored in the backup.
                 type: integer
                 example: 42
               etag:

specification/modules/end_to_end_encryption.rst

@@ -786,6 +786,7 @@ clients can discover the backup by calling ``GET /room_keys/version``.  Keys
 are encrypted according to the backups ``auth_data`` and added to the backup by
 calling ``PUT /room_keys/keys?version=$v`` or one of its variants, and can be
 retrieved by calling ``GET /room_keys/keys?version=$v`` or one of its variants.
+Keys can only be written to the most recently created version of the backup.
 Backups can also be deleted using ``DELETE /room_keys/version``, or individual
 keys can be deleted using ``DELETE /room_key/keys?version=$v`` or one of its
 variants.