sysctl.3: Reference kinfo_getvmmap(3), kinfo_getfile(3), and kinfo_getproc(3)

It seems to be a good place to help the user discover those useful
wrappers.

Reviewed by: christos
MFC after: 3 days
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51654