Document the format of the system crontab file, since cron(8) says that we do.

external/bsd/cron/dist/crontab.5

@@ -1,4 +1,4 @@
-.\"	$NetBSD: crontab.5,v 1.2 2010/05/06 18:53:17 christos Exp $
+.\"	$NetBSD: crontab.5,v 1.3 2010/07/15 22:18:20 christos Exp $
 .\"
 .\"/* Copyright 1988,1990,1993,1994 by Paul Vixie
 .\" * All rights reserved
@@ -19,7 +19,7 @@
 .\"
 .\" Id: crontab.5,v 2.4 1994/01/15 20:43:43 vixie Exp
 .\"
-.Dd April 4, 2009
+.Dd July 15, 2010
 .Dt CRONTAB 5
 .Os
 .Sh NAME
@@ -244,9 +244,21 @@ Use the first three letters of the particular day or month (case
 doesn't matter).
 Ranges or lists of names are not allowed.
 .Pp
+If the
+.Nm
+file is the system crontab
+.Pa /etc/crontab ,
+then the next (
+.Dq sixth )
+field contains the username to run the command as.
+.Pp
 The
 .Dq sixth
-field (the rest of the line) specifies the command to be run.
+field (or the
+.Dq seventh
+one for
+.Pa /etc/crontab )
+(the rest of the line) specifies the command to be run.
 The entire command portion of the line, up to a newline or percent
 signs
 .Pq Sq % ,