Add (allow-none) annotation to add_room_tag’s content

Although the specification says it is required, the body of the request
can actually be an empty JSON object; and that is what we send if a JSON
content is NULL.

src/matrix-api.c

@@ -2112,7 +2112,8 @@ matrix_api_delete_room_tag(MatrixAPI *api,
  * @user_id: the ID of the user to add the tag for
  * @room_id: the ID of the room to add the tag for
  * @tag: the tag to add
- * @content: (transfer full): extra data for the tag, e.g. ordering
+ * @content: (transfer full) (allow-none): extra data for the tag,
+ *           e.g. ordering
  * @error: return location for a #GError, or %NULL
  *
  * Add a tag to the room.