docs: Clarify return/error behaviour of D-Bus signal subscriptions
docs: Clarify return/error behaviour of D-Bus signal subscriptions

Based on a patch by David Sommerseth, from
https://gitlab.gnome.org/GNOME/glib/merge_requests/285/.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

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