Document that some config options for the user directory are in violation of the Matrix spec (#18548)

Fix #17534

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

schema/synapse-config.schema.yaml

@@ -4719,8 +4719,15 @@ properties:
       enabled:
         type: boolean
         description: >-
-          Defines whether users can search the user directory. If false then
+          Defines whether users can search the user directory. If `false` then
           empty responses are returned to all queries.
+
+
+          *Warning: While the homeserver may determine which subset of users are
+          searched, the Matrix specification requires homeservers to include (at
+          minimum) users visible in public rooms and users sharing a room with
+          the requester. Using `false` improves performance but violates this
+          requirement.*
         default: true
       search_all_users:
         type: boolean