VS: Clarify Visual Studio product year for each version

Add to the brief documentation of the Visual Studio 10, 11, and 12
generators the corresponding VS product year.  Clarify that VS11 is for
Visual Studio 2012, and VS12 is for Visual Studio 2013.
Merge topic 'vs-generator-brief-doc-update' into next

52f1464 VS: Clarify Visual Studio product year for each version