Documentation cleanup for include/clang/AST/ExprCXX.h.

This is mostly Doxygen formatting, but also updates some C++0x references
to C++11 and clarifies some wording.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185798 91177308-0d34-0410-b5e6-96231b3b80d8