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

lib/libc/gen/getgrouplist.3

@@ -1,5 +1,13 @@
+.\"-
+.\" SPDX-License-Identifier: BSD-3-Clause
+.\"
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
+.\" Copyright (c) 2025 The FreeBSD Foundation
+.\"
+.\" Portions of this documentation were written by Olivier Certner
+.\" <olce@FreeBSD.org> at Kumacom SARL under sponsorship from the FreeBSD
+.\" Foundation.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
@@ -25,12 +33,12 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 26, 2014
+.Dd August 29, 2025
 .Dt GETGROUPLIST 3
 .Os
 .Sh NAME
 .Nm getgrouplist
-.Nd calculate group access list
+.Nd produce a user's effective group list
 .Sh LIBRARY
 .Lb libc
 .Sh SYNOPSIS
@@ -40,16 +48,16 @@
 .Sh DESCRIPTION
 The
 .Fn getgrouplist
-function reads through the group file and calculates
-the group access list for the user specified in
-.Fa name .
-The
+function reads through the group database to retrieve the supplementary groups
+for the user specified in
+.Fa name ,
+and returns the effective group list, whose first group is the value of
 .Fa basegid
-is automatically included in the groups list.
-Typically this value is given as
-the group number from the password file.
+and the others are the retrieved supplementary groups.
+.Fa basegid
+typically is the user's group number from the password database.
 .Pp
-The resulting group list is returned in the array pointed to by
+The effective group list is returned in the array pointed to by
 .Fa groups .
 The caller specifies the size of the
 .Fa groups
@@ -70,6 +78,7 @@ Here, the group array will be filled with as many groups as will fit.
 group membership list
 .El
 .Sh SEE ALSO
+.Xr setcred 2 ,
 .Xr setgroups 2 ,
 .Xr initgroups 3
 .Sh HISTORY