[docs] Update the docs to remove my hedging about C++98 vs. C++11. =]
The switch has been thrown. While I'm still watching for any failures or
problems with this, the documentation can go ahead and move forward.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202566 91177308-0d34-0410-b5e6-96231b3b80d8