lock(1): Add EXAMPLES section

Add simple example showing the use of the flags: p, t, v

Reviewed by:	gbe@, yuripv@
Approved by:	manpages (yuripv@)
Differential Revision:		https://reviews.freebsd.org/D27541

usr.bin/lock/lock.1

@@ -28,7 +28,7 @@
 .\"	@(#)lock.1	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd July 10, 2002
+.Dd December 11, 2020
 .Dt LOCK 1
 .Os
 .Sh NAME
@@ -73,6 +73,11 @@ or
 .Xr vt 4
 virtual terminal.
 .El
+.Sh EXAMPLES
+Lock the terminal for 5 minutes, disable switching virtual terminals and
+require the user's login password to unlock:
+.Pp
+.Dl $ lock -p -t 5 -v
 .Sh SEE ALSO
 .Xr vidcontrol 1 ,
 .Xr syscons 4 ,