Apply LWG 2017.  This is a only a documentation change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191140 91177308-0d34-0410-b5e6-96231b3b80d8
Apply LWG 2021.  This is only a documentation change.  Also bringing c++1y status page up to date.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191141 91177308-0d34-0410-b5e6-96231b3b80d8