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

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