Doxygen documentation for version.h

This allows removing one suppression from the new checker, and it is
anyways nice that the documentation can actually be linked from relevant
parts of earlier documentation.  Some of the mentions of 4.6 could
perhaps be removed as obsolete in the future, but left them there for
now.

It requires extra investigation how to make the Doxygen build pick the
file up with an out-of-source build (if at all possible).

Change-Id: I196e89d1193cb8034580aee7d0d0bd5196ac176d