Documentation cleanup:

* Add \file documentation;
* Add \verbatim...\endverbatim markup as needed;
* Add \brief summaries;
* Escaped "::" in Doxygen comments when preceded by space, to avoid a
  Doxygen warning where Doxygen takes this as an explicit link request;
* Add \code...\endcode markup to code examples;
* Fix a grammatical glitch in "is this declarator is a".

llvm-svn: 158977