Documentation cleanup for DeclCXX.h:
* Fixing up \brief summaries (adding some, making some briefer);
* Standardizing on \commands, not @commands;
* Update C++0x references to C++11;
* Fix typos and Doxygen warnings.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186056 91177308-0d34-0410-b5e6-96231b3b80d8