Add a configuration to exclude rooms from sync response (#12310)

docs/sample_config.yaml

@@ -539,6 +539,15 @@ templates:
   #
   #custom_template_directory: /path/to/custom/templates/
 
+# List of rooms to exclude from sync responses. This is useful for server
+# administrators wishing to group users into a room without these users being able
+# to see it from their client.
+#
+# By default, no room is excluded.
+#
+#exclude_rooms_from_sync:
+#    - !foo:example.com
+
 
 # Message retention policy at the server level.
 #