Regenerate documentation post r256915:

Stop conflating WITHOUT_CLANG with WITHOUT_CLANG_IS_CC. This allows
bootstrapping a copy of clang without building clang for the base system
which is useful for nanobsd and similar setups. It's still probably
wrong to conflate what is installed as /usr/bin/cc with the selection
of a bootstrap compiler under WITH*_CLANG_IS_CC, but that's for another
day.