gsignal: Document memory management best practices for signal handlers

It’s quite common to see naked g_signal_connect() calls without a paired
g_signal_handler_disconnect(). This is commonly a bug which could lead
to uses of the callback user data after it’s been freed.

Document the best practices for avoiding this kind of bug by properly
disconnecting all signal handlers.

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