Fix Doxygen issues:

Fix Doxygen issues:
* wrap code blocks in \code ... \endcode;
* refer to parameter names in paragraphs correctly (\arg is not what most
  people want -- it starts a new paragraph);
* use \param instead of \arg to document parameters in order to be consistent
  with the rest of the codebase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163902 91177308-0d34-0410-b5e6-96231b3b80d8