docs: Add G_ENABLE_DIAGNOSTIC
docs: Add G_ENABLE_DIAGNOSTIC

We need to mention this environment variable, so that developers have a
fighting chance of actually using it.

https://bugzilla.gnome.org/show_bug.cgi?id=732184

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib