renice(8): Clarify "who" parameters.

There was a question raised in freebsd-questions@ mail list[1] about the
"who" parameters in this man page. It seems OpenBSD[2] amd NetBSD[3]
both have more legible descriptions so I borrowed some of their ideas to try
and make this page clearer.

[1] https://lists.freebsd.org/pipermail/freebsd-questions/2020-October/291914.html
[2] https://man.bsd.lv/renice
[3] https://man.netbsd.org/renice.8

Approved by: manpages (bcr@)
Differential Revision: https://reviews.freebsd.org/D26950