docs: Note need to log out and in again on push notifs setup.

This often surprises people, so mention it up front.

(Also it'd probably be good to add some code to make this step
unnecessary.)

docs/production/mobile-push-notifications.md

@@ -32,6 +32,10 @@ follows:
    Note that if you installed Zulip older than 1.6, you'll need to add
    the line (it won't be there to uncomment).
 
+4. If you or your users have already set up the Zulip mobile app,
+   you'll each need to log out and log back in again in order to start
+   getting push notifications.
+
 That should be all you need to do!
 
 If you'd like to verify the full pipeline, you can do the following.