doc: Clarify debug output settings
doc: Clarify debug output settings

Add note about G_MESSAGES_DEBUG

Pelagicore/Media-Manager-Rygel-patched,jku/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,GNOME/rygel,Tarnyko/rygel,jku/rygel,GNOME/rygel