batches: document archive behaviour
Created by: courier-new
Closes #33628 (closed).
#TeamKellaji is at it again!
- Updates out-of-date references to when changeset detaching happens
- Adds more details to what archiving is on the how-to guide "Updating a batch change"
- Adds a section with instructions for how to unarchive a changeset on the how-to guide "Updating a batch change"
- Adds an FAQ entry about archiving changesets
Test plan
Just docs changes.