Various fixes to make gtkdoc-check pass on glib

With this patchset, and a fix in gtk-doc (gtk-doc!32 (merged)) I got gtkdoc-check pass for glib (not gobject/gio yet).

Our CI really should run gtkdoc-check, because this regress all the time. I added support for that in Meson: https://github.com/mesonbuild/meson/pull/5591.