Update presence_router_callbacks.md

docs/modules/presence_router_callbacks.md

@@ -4,8 +4,10 @@ Presence router callbacks allow module developers to define additional users
 which receive presence updates from local users. The additional users
 can be local or remote.
 
-For example, it could be used to direct all of `@alice:example.com` (a local user)'s presence updates to
-`@bob:matrix.org` (a remote user), even though they don't share a room.
+For example, it could be used to direct all of `@alice:example.com` (a local user)'s
+presence updates to `@bob:matrix.org` (a remote user), even though they don't share a
+room. (Note that those presence updates might not make it to `@bob:matrix.org`'s client
+unless a similar presence router is running on that homeserver.)
 
 Presence router callbacks can be registered using the module API's
 `register_presence_router_callbacks` method.