[docs] Tweak discussion of BSDs based on feedback from Roman Divacky.
FreeBSD 10.0 and newer have a modern Clang toolchain that should work
well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202488 91177308-0d34-0410-b5e6-96231b3b80d8