Add obsolesence reasons (and comments?) to obsoletes table?

Add obsolesence reasons (and comments?) to obsoletes table to make it a bit more informative? Perhaps even aggregated xrefs?

I think for the obsoleted comment the following fields would be interesting:

- obsolescence reason
- consider
- consider label
- replaced by
- replaced by label (if possible)
- comment
- comment (often, a lot of interesting stuff is hidden in there!)
- xrefs

generally, 1 row per obsolete class, and where there are multiple values of the attributes above, just roll them into a pipe-separated string.