Remove mutual_rooms `update_user_directory` check, and add extra documentation (#12038)

Resolves #10339

changelog.d/12038.misc

@@ -0,0 +1 @@
+Remove check on `update_user_directory` for shared rooms handler (MSC2666), and update/expand documentation.

docs/workers.md

@@ -528,10 +528,19 @@ the following regular expressions:
 
     ^/_matrix/client/(r0|v3|unstable)/user_directory/search$
 
-When using this worker you must also set `update_user_directory: False` in the
+When using this worker you must also set `update_user_directory: false` in the
 shared configuration file to stop the main synapse running background
 jobs related to updating the user directory.
 
+Above endpoint is not *required* to be routed to this worker. By default,
+`update_user_directory` is set to `true`, which means the main process
+will handle updates. All workers configured with `client` can handle the above
+endpoint as long as either this worker or the main process are configured to
+handle it, and are online.
+
+If `update_user_directory` is set to `false`, and this worker is not running,
+the above endpoint may give outdated results.
+
 ### `synapse.app.frontend_proxy`
 
 Proxies some frequently-requested client endpoints to add caching and remove

synapse/rest/client/mutual_rooms.py

@@ -42,17 +42,19 @@ class UserMutualRoomsServlet(RestServlet):
         super().__init__()
         self.auth = hs.get_auth()
         self.store = hs.get_datastores().main
-        self.user_directory_active = hs.config.server.update_user_directory
+        self.user_directory_search_enabled = (
+            hs.config.userdirectory.user_directory_search_enabled
+        )
 
     async def on_GET(
         self, request: SynapseRequest, user_id: str
     ) -> Tuple[int, JsonDict]:
 
-        if not self.user_directory_active:
+        if not self.user_directory_search_enabled:
             raise SynapseError(
                 code=400,
-                msg="The user directory is disabled on this server. Cannot determine shared rooms.",
-                errcode=Codes.FORBIDDEN,
+                msg="User directory searching is disabled. Cannot determine shared rooms.",
+                errcode=Codes.UNKNOWN,
             )
 
         UserID.from_string(user_id)