ddb(4): document some missing commands

This is not exhaustive, just what I collected while working on
mac_ddb(4).

Reviewed by: pauamma, markj, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation (in part)
Differential Revision: https://reviews.freebsd.org/D37333

(cherry picked from commit 831979ae0fb91a2ff674b7b1ab7d2c252a17e27d)