On .deb systems you will need to install the package "gtk-doc-tools", and then run
./autogen.sh
Reference generation is disabled by default but you can enable it with :
./configure --enable-gtk-doc
Then you can read the reference at docs/reference/denemo/html/index.html
It does not have a lot of information now because the code is mostly uncommented. But this can motive us to comment a bit more. The syntax of the comments used by gtk-doc is very similar to doxygen syntax, and can be read here :
https://wiki.gnome.org/DocumentationProject/GtkDoc#Documenting_a_section