Documentation cleanup: fixing documentation for FrontendAction.

* Fix a typo, s/BeginSourceAction/BeginSourceFile/, so that the documentation
  for FrontendAction::BeginSourceFileAction links correctly to BeginSourceFile;
* Add some basic \file documentation for FrontendAction.h;
* More use of "\brief" instead of repeating the name of the entity being
  documented;
* Stop using Doxygen-style "///" comments in FrontendAction.cpp, as they were
  polluting the documentation for BeginSourceFile;
* Drop incorrect "\see" markup that broke Doxygen's formatting;
* Other minor documentation fixes.

llvm-svn: 173213