gsocketlistener: Clarify documentation of add_any_inet_port()

It wasn’t clear what the fallback behaviour of IPv4 vs IPv6 was, or that
the same port was used for both.

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

gio/gsocketlistener.c

@@ -1060,6 +1060,14 @@ g_socket_listener_close (GSocketListener *listener)
  * This is useful if you need to have a socket for incoming connections
  * but don't care about the specific port number.
  *
+ * If possible, the [class@Gio.SocketListener] will listen on both IPv4 and
+ * IPv6 (listening on the same port on both). If listening on one of the socket
+ * families fails, the [class@Gio.SocketListener] will only listen on the other.
+ * If listening on both fails, an error will be returned.
+ *
+ * If you need to distinguish whether listening on IPv4 or IPv6 or both was
+ * successful, connect to [signal@Gio.SocketListener::event].
+ *
  * @source_object will be passed out in the various calls
  * to accept to identify this particular source, which is
  * useful if you're listening on multiple addresses and do