login_cap(3): Document login_getcapenum()

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40685

(cherry picked from commit 97256feb7a65cccac51a03846c5308894a2cb445)

Approved by: markj (mentor)