Add freeze/thaw description to devctl(8)

This is a follow-up to 5fa29797910346fc0c54829bd979856e83b9b7ea .

PR: 256311
Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29867

(cherry picked from commit 315674fb6acc4fa54cf82de3863c349c5a71f498)