capsicum.4: Add some more detail from the Capsicum paper

Adapt some language from "Capsicum: practical capabilities for UNIX"
https://www.cl.cam.ac.uk/research/security/capsicum/papers/2010usenix-security-capsicum-website.pdf

Reviewed by: markj
Discussed with: rwatson
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50855

(cherry picked from commit c54534e60263954fac4db5ebb041ed4597df1195)
(cherry picked from commit ce65ff203a4f7ce59c8782f87d90a657f7ccbc46)