Update sa(4) comments and man page after review.

sys/cam/scsi/scsi_sa.c:
Add comments explaining the priority order of the various
sources of timeout values. Also, explain that the probe
that pulls in drive recommended timeouts via the REPORT
SUPPORTED OPERATION CODES command is in a race with the
thread that creates the sysctl variables. Because of that
race, it is important that the sysctl thread not load any
timeout values from the kernel environment.

share/man/man4/sa.4:
Use the Sy macro to emphasize thousandths of a second
instead of capitalizing it.

Requested by: Warner Losh <imp@freebsd.org>
Requested by: Daniel Ebdrup Jensen <debdrup@freebsd.org>
Sponsored by: Spectra Logic
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33883