In order to activate reference documentation, you have to specify
the following configure command line:
$ ./configure --enable-gtk-doc --enable-gtk-doc-html

Then, cd to doc/reference and launch make command.

(This directory is not recursively make when launching make from top
directory. So, this avoid spending time in reference documentation
computation when not really needed.)

Unfortunately to regenerate the references running make again does not
detect any changes to the source files, thus it must be regenerated
manually:

$ make clean ; make

If you want remove the generated HTML files you need to perform a more
thorough cleanse:

$ make distclean

Unfortunately this also removes the Makefile, so you have to go through
the ./configure steps again to regenerate the Makefile.

