cap_enter(2): fix CAVEATS section

The CAVEATS section was misspelled as "CAVEAT" before this change. Fix the
spelling to identify issues related to the section.

Furthermore, given that the section order was incorrect, move the CAVEATS
section down to the bottom of the manpage, per the conventional section
order.

MFC after: 1 week
Reported by: make manlint
Sponsored by: DellEMC Isilon

(cherry picked from commit 20daf0ca6ea8ac82fa3a88f1d5e68507773c9644)