window: Add a note about the trustworthiness of the client PID

Since PIDs are inherently insecure because they are reused after a
certain amount of processes was started, it's possible the client PID
was spoofed by the client.

So make sure users of the meta_window_get_pid() API are aware of those
issues and add a note to the documentation that the PID can not be
totally trusted.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1180

src/core/window.c

@@ -7590,6 +7590,9 @@ meta_window_get_transient_for (MetaWindow *window)
  * Returns the pid of the process that created this window, if available
  * to the windowing system.
  *
+ * Note that the value returned by this is vulnerable to spoofing attacks
+ * by the client.
+ *
  * Return value: the pid, or 0 if not known.
  */
 pid_t