For example the gnome people use the doc use flag to control whether
gtk-doc gets rebuild using cross references:

16:51 <@leio> as far as I'm concerned the doc USE flag means rebuilding
documentation to get cross-referencing in docs working
16:51 <@leio> also the lack of doc USE flag does not mean to not install
documentation
16:52 <@leio> it means to not take a long time to build documentation,
and we are not doing it if doc USE flag is not present

This leads to having tons of gtk-doc installed:
betelgeuse@pena ~ $ du -sh /usr/share/gtk-doc/html/
51M     /usr/share/gtk-doc/html/

In for example Java we use it to control Javadoc installation. Javadoc
generation rarely takes much time and needs no extra dependencies but
having Javadocs for everything would consume a lot of space.

My opinion is to make it clear that the doc use flag always controls
whether or not to install documentation and make it clear in the
devmanual. For what gnome does, they can then add for example a gtk-doc
use flag to control the building of the cross references and have the
doc use flag control the installation of the bundled documentation.

betelgeuse@pena ~ $ euse -i doc
global use flags (searching: doc)
************************************************************
[-    ] doc - Adds extra documentation (API, Javadoc, etc)

INSTALL_MASK is of course a solution to not installing gtk-doc at all
but it doesn't give me the ability to install it only for individual
packages.

What do others think?

Regards,
Petteri