Update docs to reflect new minimum MSVC version requirement

Mailing list discussion about this:
http://lists.llvm.org/pipermail/llvm-dev/2016-September/104631.html

Code changes to simplify the ifdefs will come next, and can be reverted
without affecting the policy if someone needs it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284660 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed