cam/da: Document what an 'invalid pack' means.

An 'invalid pack' in the da driver means that the media for the drive is
temproarily missing, pending an action by a third party (eg robot arm or
human inserting a disk pack, etc) or has changed since we opened. We
invalidate a pack when the ASC/ASCQ tells us the media is missing (ASC
0x3a MEDIA MISSING), for 0x28/0 (NOT READY TO READY CHANGE, MEDIUM MAY
HAVE CHANGED) and for that brief period of time after a peripheral has
been invalidated before the disk is destroyed. When the pack is invalid,
we can't do READ/WRITE operations to the media, so we fail all
outstanding I/O and any new I/O that comes in when this happens. This
is cleared in daopen when we've repobed the device.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D48687