Documentation of the fields exported

[mikegerber]

(Moved from #26)

There's also the issue that I usually forget what we already included in our current mods4pandas/current mods_info export. We should have a complete description of the fields. This should ideally be generated automatically, or at least checked for completeness. This would also enable checking what changed between versions of the export. (This is related #26 because I simply could not tell @labusch what currently is possible with the info we already export.)