gmain: Clarify that g_source_set_callback() is safe on attached sources
gmain: Clarify that g_source_set_callback() is safe on attached sources

g_source_set_callback() and g_source_set_callback_indirect() are both
safe to call zero or more times on attached sources. The change in
callback will take effect the next time the source is dispatched, after
the set_callback() call returns (it could block due to locking).

https://gitlab.gnome.org/GNOME/glib/issues/827

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