freebsd-update.8: Fix use of mdoc(8) macros

- Use Ar for arguments.
- Use Cm for freebsd-update commands like "upgrade".
- Reference other manuals with Xr when possible.

MFC after:	1 week

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

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 1, 2021
+.Dd March 29, 2022
 .Dt FREEBSD-UPDATE 8
 .Os
 .Sh NAME
@@ -43,7 +43,7 @@
 .Op Fl s Ar server
 .Op Fl t Ar address
 .Op Fl -not-running-from-cron
-.Cm command ...
+.Ar command ...
 .Sh DESCRIPTION
 The
 .Nm
@@ -106,7 +106,8 @@ Trust an RSA key with SHA256 of
 .It Fl r Ar newrelease
 Specify the new release (e.g., 11.2-RELEASE) to which
 .Nm
-should upgrade (upgrade command only).
+should upgrade
+.Pq Cm upgrade No command only .
 .It Fl s Ar server
 Fetch files from the specified server or server pool.
 (default: read value from configuration file.)
@@ -119,11 +120,14 @@ command, if any, to
 .It Fl -not-running-from-cron
 Force
 .Nm Cm fetch
-to proceed when there is no controlling tty.
+to proceed when there is no controlling
+.Xr tty 4 .
 This is for use by automated scripts and orchestration tools.
 Please do not run
 .Nm Cm fetch
-from crontab or similar using this flag, see:
+from
+.Xr crontab 5
+or similar using this flag, see:
 .Nm Cm cron
 .It Fl -currently-running Ar release
 Do not detect the currently-running release; instead, assume that the
@@ -190,7 +194,9 @@ If your clock is set to local time, adding the line
 .Pp
 .Dl 0 3 * * * root /usr/sbin/freebsd-update cron
 .Pp
-to /etc/crontab will check for updates every night.
+to
+.Pa /etc/crontab
+will check for updates every night.
 If your clock is set to UTC, please pick a random time
 other than 3AM, to avoid overly imposing an uneven load
 on the server(s) hosting the updates.