nvme: Add comments about other fields in status

When manually completing an I/O, we do so because we have no status back
from the card. Note M, CRD and P are all 0 because this is an artificial
event (and phase isn't checked when it's completed this way). There's no
MORE information in the error log page and there's no delayed retry
(CRD=0) and we don't currently request CRD to be set to anything other
than 0 and thus don't implement delayed retry.

Sponsored by: Netflix
Reviewed by: chuck, mav, jhb
Differential Revision: https://reviews.freebsd.org/D41314