Updates to Versioning finding

I did a modest rewrite of section 5.1 and onwards on forwards compatible
versioning.  I've tried to make the decisions about whether to discard or
preserve and how much to discard or ignore more explicit, including a simple
venn diagram.  I also cleaned up the version numbers section.

http://www.w3.org/2001/tag/doc/versioning-compatibility-strategies-20081205.html
http://www.w3.org/2001/tag/doc/versioning-compatibility-strategies-20081205.xml

Cheers,
Dave