(nice invocation): Document the "nice value", and

(nice invocation): Document the "nice value", and
how it affects the scheduling priority.  (The old documentation
implied that the nice value equaled the scheduling priority, which
isn't accurate.)  Document that the range of nice values might
exceed -20..19.  Specify what happens when you give a nice value
that is out of range, or when you don't have permissions to lower
the nice value.  Bash doesn't have a builtin 'nice', so don't say
"most shells" have one.