docs: use %, #, and @ more consistently for contants, types, and args

https://bugzilla.gnome.org/show_bug.cgi?id=731182
docs: Assorted fixes and additions to reduce gtk-doc warnings

https://bugzilla.gnome.org/show_bug.cgi?id=731182
introspection: add many missing nullable annotations

https://bugzilla.gnome.org/show_bug.cgi?id=731182