ktrace: Describe CAPFAIL trace point in man page

Update the ktrace(1) man page to describe the recently improved
capability failure tracing.

Approved by: markj (mentor)
Reviewed by: markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D44886

(cherry picked from commit 4f2ada0d88219746fdfbc0907c28b0950ad18725)