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.

llvm-svn: 159616