Better copyright advice

Document the common practices around copyrights with "all rights reserved" in
them as new copyright notices get added.

It's an open question qhether to point people at the fact that since the Berne
convention was ratified, All rights reserved is largely obsolete.
https://en.wikipedia.org/wiki/All_rights_reserved#Obsolescence has the
details. The committer's guide will be revised shortly, and it's likely that's a
better place for this discussion. If not, I'll add a blurb here.

Reviewed by: jhb@, brooks@
Differential Review: https://reviews.freebsd.org/D22800