better doc

synapse/config/user_directory.py

@@ -32,9 +32,12 @@ class UserDirectoryConfig(Config):
     def default_config(self, config_dir_path, server_name, **kwargs):
         return """
         # User Directory configuration
+        #
         # 'search_all_users' defines whether to search all users visible to your HS
         # when searching the user directory, rather than limiting to users visible
-        # in public rooms. Defaults to false.
+        # in public rooms.  Defaults to false.  If you set it True, you'll have to run
+        # UPDATE user_directory_stream_pos SET stream_id = NULL;
+        # on your database to tell it to rebuild the user_directory search indexes.
         #
         #user_directory:
         #   search_all_users: false