A couple years ago I wrote a small tip about how to integrate the devhelp installed by your distribution with the documentation generated through jhbuild.
Well, that way is plain wrong. I noticed it when I upgraded to GNOME 3: modifying the environment variable XDG_DATA_DIRS, the gnome-session goes crazy.
So, I found a less intrusive way to make your distro-based devhelp reads the jhbuild-generated documentation:
ln -s /opt/jhbuild/share/gtk-doc/ ~/.local/share/
And that is it!