Remove references to pdwait4(2) and CAP_PDWAIT from rights(4)

@cem removed references to pdwait4(2) (a nonexistent syscall) in
r320058.

This change removes references to pdwait4(2) and CAP_PDWAIT in
rights(4) to not mislead the user into thinking that pdwait4(2)/CAP_PDWAIT is
actually implemented in the stock FreeBSD kernel.

The goal of this functionality was to simplify monitoring/manipulating
processes started with pdfork, et al, and avoid races with waiting on pids.
The syscall was never completed though--just discussed on the capsicum mailing
list back in 2015:
https://lists.cam.ac.uk/pipermail/cl-capsicum-discuss/2015-May/msg00012.html
. That being said, there are members of the project (@rwatson, etc) who
have longterm goals to implement this syscall to better secure pdfork(2)
calls.

PR: 235871
Reviewed by: emaste
Discussed with: rwatson
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18950