KASSERT(9): add assertion message guidelines

Add some text describing how to create useful assertion messages.
Improve and add to the EXAMPLES.

See the discussion prompting this on -hackers:
https://mail-archive.freebsd.org/cgi/mid.cgi?57o4rnnq-013s-3nsn-59n5-4ssn1pq81s94

Reviewed by: emaste
Discussed with: imp, bz
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44434