login.conf(5): Document priority's default and possible values

Priority is reset to 0 if not explicitly specified.

While here, be more explicit about what "Initial priority (nice) level"
means and document that it is possible to set real-time or idle class'
priorities with this capability.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40689

(cherry picked from commit aa96945e71b0f724e6ce673194d23f58d2d637d3)

Approved by: markj (mentor)