Documentation fixes - the rest of DeclCXX.h:
* More \brief additions/fixes;
* Fix some misleading comments about C++11's explicit conversion operators;
* Mark up some \code examples;
* Add \file documentation.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186059 91177308-0d34-0410-b5e6-96231b3b80d8