In general... Spack packages should not build documentation unless asked to do so (eg. with a +docs variant). Reasons:

1. Building docs can involve large/annoying dependencies (doxygen, latex, etc.)
2. Users don't usually want docs except for packages near the top of the DAG for a particular user's needs. CGAL might be a great and clever system; but people installing stuff five levels up just don't care.
3. Even if a packages is close to the top, users often just rely on on-line docs anyway.

I'm guilty of not following this advice (need to fix for glint2 and icebin packages).