KASSERT(9): describe history of MPASS

The macro originates from BSD/OS, with a different etymology than what
is presented. Add a brief HISTORY section to capture this.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44439

(cherry picked from commit 5d956e11ed3713ff769d51b71f7b4ee6ce0085fe)