Add wording about the priority range and mention what effect this has at certain values.
Add wording about the priority range and
mention what effect this has at certain
values.

PR:		124469
Obtained from:	NetBSD nice.1 v1.14
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase