gmessages: Update advice for G_LOG_DOMAIN
gmessages: Update advice for G_LOG_DOMAIN

Suggest defining it for all code — for applications as well as for
libraries. This allows G_MESSAGES_DEBUG=my-app to be used to filter out
all messages from libraries which it uses, for example.

https://bugzilla.gnome.org/show_bug.cgi?id=777956

endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib