Clarify MXC URI string formats in m.room.avatar

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/event-schemas/schema/core-event-schema/msgtype_infos/avatar_info.yaml

@@ -22,6 +22,8 @@ properties:
     description: |-
       The URL (typically [`mxc://` URI](/client-server-api/#matrix-content-mxc-uris)) to a thumbnail of the image.
     type: string
+    format: mx-mxc-uri
+    pattern: "^mxc:\\/\\/"
   thumbnail_info:
     allOf:
       - $ref: thumbnail_info.yaml

data/event-schemas/schema/m.room.avatar.yaml

@@ -16,6 +16,8 @@ properties:
           The URL to the image. If this property is not present, the room has no avatar. This can be useful
           to remove a previous room avatar.
         type: string
+        format: mx-mxc-uri
+        pattern: "^mxc:\\/\\/"
     type: object
   state_key:
     description: A zero-length string.