Expand description
The values a write is allowed to take.
Everything else in this tool answers what is; this answers what may be.
issue create --type and issue update --set priority=… are otherwise
written on faith and judged by Tracker, which refuses with a message about a
field the caller was never shown a list for.
Read-only, and organisation-wide: a queue narrows these — queue get says
which type and priority its issues start with — but the dictionary itself is
defined once for the whole organisation.