realgroupmember(9): Link to groupmember(9), document the function

Reviewed by: bcr (older version), mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40643

(cherry picked from commit 5d9f38405a10fdcd9fc108c940dcf2642e9f1833)

Approved by: markj (mentor)