Documentation cleanup for clang-c:

* Add parameter names to various \param commands;
* Ensure that \brief summaries are followed by blank lines where needed to
  prevent subsequent text being merged into the summary;
* Mark code examples with \code...\endcode so that they're appropriately
  formatted by Doxygen.
* Add qualification for links to field of structs (specifically callbacks);
* Escape #, @ and < as needed to avoid them being interpreted by Doxygen;
* Remove incorrect uses of "\see name", preferring #name;
* Fix a glitchy "\param Returns..." to be "\returns";
* Fix parameter names given to \param, typos and otherwise.

llvm-svn: 158489