capsicum.4: Add capsicum_helpers and libcasper references

These were present in the SEE ALSO section but were not explicitly
mentioned. Add a brief description of both.

Reviewed by: ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50707

(cherry picked from commit 6d46fd2cbea43002689ae1a8a0e388e9774da1bc)