gmain: Document memory management best practices for GSources

It’s very common to see code where a timeout is scheduled using
g_timeout_add(), yet the owning object could be destroyed shortly
afterwards, before the timeout is fired, leading to use-after-free.

Try and prevent this happening with new code by documenting best
practices for memory management of user data for GSource callbacks.

https://bugzilla.gnome.org/show_bug.cgi?id=741779