Add caveat to kinfo_getvmmap(3) explaining high CPU utilisation.
Based on kib's reply on https://lists.freebsd.org/pipermail/freebsd-hackers/2016-July/049710.html

PR: 210904
Submitted by: Yuri Victorovich <yuri AT rawbw DOT com>
Reviewed by: jilles
Approved by: bcr (mentor)
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D12004