📚 Document COPYUID in tagged vs untagged responses

[nevans]

COPYUID was originally (in the UIDPLUS extension) only allowed to be sent in the tagged response to the command which copied the messages.

Because the move commands atomically combine copy with expunge, the MOVE extension changed that for move commands, to recommend sending it as an untagged response prior to the tagged response for the move. This way, EXPUNGE responses could also be sent prior to the completion of the MOVE. IMAP4rev2 updates that recommendation to a requirement.

However, some of the biggest email providers (most notably, GMail) follow the original UIDPLUS extension's requirement and still send the COPYUID response with the tagged response, even for move commands.