dmesg: Better wording from review

I pushed the last changes before I'd noticed the better wording
suggestions in the review. Also include a note that not all lines will
have a timestampe. Some multi-line messages are generated with sbuf, and
when those are pushed, only the first line will have the
timestamp. Document this quirky behavior as well since fixing it likely
won't happen soon. CAM periph drivers generate all the lines in their
announce message together so they aren't intermingled with other things,
for example.

Suggested by: allanjude, emaste, rpokala
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35139
Differential Revision: https://reviews.freebsd.org/D35141