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