style(9): Add advice about $FreeBSD$

Codify our standard practice with $FreeBSD$
o New code only needs it if it might land in stable/12
o Old code should retain it until stable/12 is unsupported
o We'll do a bulk remove in the future: don't do it proactively.
o Give advice about how to tag files derived from other files

in the tree.

Reviewed by: bcr, allanjude,ceri
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30789