docs: Add missing GTest functions

The GTest message API is missing from the reference.
Drop a no-longer-existing example from the docs

gapplication-example-menu.c was dropped in
0c094d660769a00564ef33a775a387f62cf2ff41, two years ago.
Time to remove its inclusion in the docs too.

https://bugzilla.gnome.org/show_bug.cgi?id=722973
updated kn.po
Merge branch 'glib-2-38' of git://git.gnome.org/glib into v2-38-stable