Add missing bigdescription list item

[mbtaylor]

There was a missing item in a bigdescription environment in Appendix A.5. This screwed up some of the TtH formatting later in the document (Lots of bold text). I've added what I think is reasonable text for the item ("QUDT"), but I'm not certain what the original intention was, so somebody better check (git blame says @nxg wrote this in the first place).

[nxg]

This looks entirely plausible – thanks