GDBusMethodInvocation: Clarify how the ownership is handled
GDBusMethodInvocation: Clarify how the ownership is handled

https://bugzilla.gnome.org/show_bug.cgi?id=767952

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