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• Updates out-of-date references to when changeset detaching happens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.