Gio::ActionMap: Add function to ActivateSlot doc
Gio::ActionMap: Add function to ActivateSlot doc

It's used in add_action_with_bool() too.

https://bugzilla.gnome.org/show_bug.cgi?id=774444

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm