Add last_seen_ts to query user example (#17976)

This section could probably do with a lot more editorial attention, but
for now this is all there is in terms of documentation. The field is
already returned by Synapse:
4587decd67/synapse/handlers/admin.py (L150)

`last_seen_ts` was introduced in
https://github.com/matrix-org/synapse/pull/16218

changelog.d/17976.doc

@@ -0,0 +1 @@
+Add previously-undocumented `last_seen_ts` to query user admin API.

docs/admin_api/user_admin_api.md

@@ -40,6 +40,7 @@ It returns a JSON body like the following:
     "erased": false,
     "shadow_banned": 0,
     "creation_ts": 1560432506,
+    "last_seen_ts": 1732919539393,
     "appservice_id": null,
     "consent_server_notice_sent": null,
     "consent_version": null,