I suggest the following diff against /src/lib/libc/sys/setgroups.2 :

@@ -57,6 +57,9 @@
 more than
 .Dv {NGROUPS_MAX} .
 .Pp
+This function does not affect the primary real and effective group of the
+process; only secondary ones are set.
+.Pp
 Only the super-user may set new groups.
 .Sh RETURN VALUES
 A 0 value is returned on success, -1 on error, with
@@ -82,7 +85,9 @@
 .El
 .Sh SEE ALSO
 .Xr getgroups 2 ,
-.Xr initgroups 3
+.Xr initgroups 3 ,
+.Xr setgid 2 ,
+.Xr setegid 2
 .Sh HISTORY
 The
 .Fn setgroups