pw: Clarify groupadd/groupmod memberlist formats

Clarify in which formats the memberlist for groupadd/groupmod can be
accepted for '-M', '-m', or '-d' flag.

Related commit:	40ab104.

Reviewed by: imp
Pull Request: #1352

usr.sbin/pw/pw.8

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd November 28, 2022
+.Dd July 29, 2024
 .Dt PW 8
 .Os
 .Sh NAME
@@ -837,7 +837,8 @@ new group
 or replace an existing membership list
 .Pq in Cm groupmod .
 .Ar memberlist
-is a comma separated list of valid and existing user names or uids.
+is a comma, space, or tab-separated list of valid and existing user names or
+uids.
 .It Fl m Ar newmembers
 Similar to
 .Fl M ,