param.h: Document __FreeBSD_version better

Document what __FreeBSD_version means a bit better by documenting the
sorts of events it should be bumped for. Also include a handy shorthand
for what it means. Add a some advice for how frequently to change this
as well.

Added a note about the approved way to parse this from the param.h file,
though that was not in the review. All in-tree users have been updated
to this method prior to this commit. Move and reword the comment that
was on the same line.

Suggestions by: greg@unrelenting, arch@
Reviewed by: rgrimes@ (earlier version).
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29850