Delete a member's local user when the portal removes them

On a member_removed frame the connector deletes the local user mapped to
that portal account, never the owner and never a superuser, since the
portal only ever creates ordinary accounts here. An instance offline at
the time keeps the inert row, as before.

docs/interface/portal.md

@@ -58,8 +58,9 @@ time-limited; generate a fresh one if it expires.
 connection error. The instance needs outbound access to the portal, and any
 proxy on that route must support WebSockets.
 
-- To remove a person's access, remove them from the portal's sharing dialog
-  or edit their account under **Admin → Users** and clear **Is active?**.
+- To remove a person's access, remove them from the portal's sharing dialog,
+  which also deletes their local account while the instance is connected, or
+  delete or deactivate their account under **Admin → Users**.
 - To end all portal access from the instance, press **Disconnect**. This stops
   the connection and invalidates portal credentials locally, without requiring
   the portal to be online.