Document the PAGER environment variable

Sometimes users want to use freebsd-update(8) in a non-interactive way and
what they often miss is that they have to set PAGER to cat(1) in order to
avoid interactive prompts from less(1).

MFC after:	4 weeks

usr.sbin/freebsd-update/freebsd-update.8

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 24, 2019
+.Dd November 14, 2020
 .Dt FREEBSD-UPDATE 8
 .Os
 .Sh NAME
@@ -194,6 +194,20 @@ it cannot be trusted to operate correctly.
 If you intend to use this command for intrusion-detection
 purposes, make sure you boot from a secure disk (e.g., a CD).
 .El
+.Sh ENVIRONMENT
+.Bl -tag -width "PAGER"
+.It Ev PAGER
+The pager program used to present various reports during the execution.
+.Po
+Default:
+.Dq Pa /usr/bin/less .
+.Pc
+.Pp
+.Ev PAGER
+can be set to
+.Dq cat
+when a non-interactive pager is desired.
+.El
 .Sh FILES
 .Bl -tag -width "/etc/freebsd-update.conf"
 .It Pa /etc/freebsd-update.conf