Documentation cleanup:

* Primarily, added \brief to most of include/clang/Basic, instead of prefixing
  the comments with "DeclaredName - ";
* Made some brief summaries significantly briefer;
* Fixed up some erroneous uses of \see and \arg;
* Fixed up some extraneous backslashes in \code...\endcode blocks;
* Fixed up some typos/spelling errors.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159616 91177308-0d34-0410-b5e6-96231b3b80d8