Collections don't have a description property in API responses and there seems to be no include flag to request them (I've tried description and annotation).

It would be great to have them in lookup responses (https://musicbrainz.org/ws/2/collection/<mbid> and https://musicbrainz.org/ws/2/collection/<mbid>/<plural-type>) at least, maybe also when browsing them by user (https://musicbrainz.org/ws/2/collection?editor=<name>). But they could be very long.