docs: Require gtk-doc 1.20
docs: Require gtk-doc 1.20

Our documentation comments were written for a version of gtk-doc that
has since been replaced by newer versions that are not quite backwards
compatible. 1.20 is the version from Ubuntu's last LTS release, so this
should be available on most systems.

wmvanvliet/Chimara,wmvanvliet/Chimara,wmvanvliet/Chimara,wmvanvliet/Chimara,wmvanvliet/Chimara