VS: Make it clear what Visual Studio edition each vs generator is for.

Add to the brief documentation of each visual studio generator what
Visual Studio edition each generator is for. I want to clarify that
VS11 is for Visual Studio 2012, and VS12 is for Visual Studio 2013.
Merge topic 'vs-generator-brief-doc-update' into next

26e728f VS: Make it clear what Visual Studio edition each vs generator is for.