Documentation on Gtk.Template

I was wondering why there is no documentation on Gtk.Template.

It is used in some places and some of the usage can be infered but it would be great to have:

https://lazka.github.io/pgi-docs/Gtk-3.0/classes/Template.html

Usually this is (mostly) autogenerated isn't it? Why is it missing? With the few tutorials I'm a little lost on using it.