math/R: Revert documentation changes
In an attempt to simplify the handling of documentation for users, changes
were made in the last commit. When the documentation knob was off,
(almost) no HTML, PDF, etc. files or their sources were installed, and when
the knob was on, all these files were installed. Because some of these
documentation files depended on some very large TeX ports, despite the
usual convention, the knob was turned off by default. Unfortunately, this
caused problems for a few R-dependent ports, because they copy
documentation files from math/R when they are installed.