getgrouplist.3: Rework, use more appropriate terminology

Bring up to date with NSS by substituting "file" with "database".

Describe more precisely which database is accessed, i.e., only the group
database, which value should be used and where it lands in the result.

Prefer a terminology referring to POSIX terms, i.e., use "effective
group list" instead of "group access list".

Reviewed by: gbe
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52283