gdbusconnection: Document main context iteration for unsubscriptions

Add a note to the documentation of
`g_dbus_connection_signal_unsubscribe()`, `g_bus_unwatch_name()` and
`g_bus_unown_name()` warning about the need to continue iterating the
caller’s thread-default `GMainContext` until the
unsubscribe/unwatch/unown operation is complete.

See the previous few commits and #1515 for an idea of the insidious bugs
that can be caused by not iterating the `GMainContext` until
everything’s synchronised.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gdbusnamewatching.c

@@ -858,6 +858,13 @@ guint g_bus_watch_name_on_connection_with_closures (
  *
  * Stops watching a name.
  *
+ * Note that there may still be D-Bus traffic to process (relating to watching
+ * and unwatching the name) in the current thread-default #GMainContext after
+ * this function has returned. You should continue to iterate the #GMainContext
+ * until the #GDestroyNotify function passed to g_bus_watch_name() is called, in
+ * order to avoid memory leaks through callbacks queued on the #GMainContext
+ * after it’s stopped being iterated.
+ *
  * Since: 2.26
  */
 void