math/R: Revert documentation changes and bump PORTREVISION

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.

Reported by: antoine
Reviewed by: mat
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8429