KASSERT(9): some updates

• Add a little bit of introductory text
• Improve the existing example: ANSI C, use a better assertion than a NULL check (which is discouraged)
• Document the widely used MPASS macro in this page
• Drop the cross-reference to config(8)

Reviewed by: kib, markj, rpokala, Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39131

(cherry picked from commit 87132d1dce4b61c782871f450c17b818bf991ff6)