Gtkmm4 documentation not installed

I’ve been developing gtkmm4 apps for awhile now, but I recently needed ot look for some documentation so naturally I installed gnome.devhelp, but even in a nix-shell (run with nix-shell -p gtkmm4), no gtkmm4 docs appear. Am I doing anything wrong and is there a way that I can install the documentation?

Hosted by Flying Circus.