login.conf(5): Document priority's special value 'inherit'

Reviewed by: emaste, yuripv (older version)
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40691

(cherry picked from commit 33d4ce5496efe96bbaa7663e298876395b1db62c)

Approved by: markj (mentor)