docs: Remove note about Identifiers being used for C++ operators

C++ operators are represented by CXXOperatorName now, which is already
documented.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191559 91177308-0d34-0410-b5e6-96231b3b80d8
docs: Document CXXLiteralOperatorName and CXXUsingDirective

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191560 91177308-0d34-0410-b5e6-96231b3b80d8