Remove an obsolete comment in devstat(3) about the accuracy of the
milliseconds per transaction (DSM_MS_PER_TRANSACTION) calculation.

The comment was accurate many years ago when the kernel didn't
record I/O times on a per-I/O basis, but now that we do collect
that information in most areas, it isn't correct.

The milliseconds per transaction values are correct, assuming the
I/O duration has been recorded.

Sponsored by: Spectra Logic
MFC after: 3 days