GSimpleAction: Clarify/fix @parameter(_type) docs
GSimpleAction: Clarify/fix @parameter(_type) docs

If something is nullable, it's always helpful to identify what NULL
means. Also, this is not the parameter for the .activate() vfunc, as we
take that over: rather, it is the parameter for the ::activate signal.

https://bugzilla.gnome.org/show_bug.cgi?id=795070

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