capsicum: briefly describe capabilities in man page

Provide a very brief introduction to capabilities, using a couple of
sentences from David Chisnall's mailing list response[1] to a question
about Linux capabilities and Capsicum.

Mailing list subject (in case the archive URL changes) was
Re: Linux capabilities to Capsicum

[1] https://lists.freebsd.org/archives/freebsd-hackers/2022-April/001032.html

Reviewed by: oshogbo
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34945

(cherry picked from commit 1f568792c6156988d357ea31a36d77ed11cc9a2d)