procctl(2): Clarify the ESRCH error code case

[arrowd]

Summary

Relevant pkg PR https://github.com/freebsd/pkg/pull/2322

[kib]

lib/libsys/procctl.2

758

Such cases are usually described by listing the error for each condition.
I.e. there should be yet another
.It Bq Er ESRCH
list element with the adapted text of your change.

[arrowd]

• Move the clarification into a separate list item

[kib]

I would say something like 'No descendant processes can be found matching criteria specified in .Dv PROC_REAP_KILL request'.

Note that this thing kills not only children, but the whole (sub)tree. I also want the language to be similar to that from killpg(2).

[arrowd]

• Rephrase