cam/da: Add some comments about 28/0 asc

We don't set the PACK_INVALID flag in response to asc 28/0 "NOT READY
TO READY CHANGE, MEDIUM MAY HAVE CHANGED". Explain why: we usally
retry this (since it's a UNIT ATTENTION) and it works and/or it's
before daopen. In addition, improve the comment about the asysnc unit
attention to make it a little more understandable. This is likely
sufficient (it's been since before FreeBSD 3.0 when this hack was put
in). Ideally, we'd verify what medium is there, but knowing when it's
the same is a bit tricky...

Differential Revision: https://reviews.freebsd.org/D48839
Sponsored by: Netflix