Add description to device properties

In the laboratory instrument meeting we discussed that it could be useful to add descriptions to measured quantities.

See https://notes.desy.de/1qPRYkNWSsO9b69fffg5iQ

(While we also discussed that it would be better to store some of the values in key-value-pairs, I still think adding some more description fields that can be used optionally is still a good decision).