Update and clarify pflog man page

The pflog0 interface is created when the module is loaded, this can
be triggered by pf and pflogd being enabled or by kldloading the module.

By default the interface would be pflog0, add the ifconfig stage of the
example to make this example clearer.

Reviewed by:	kp, bz, bcr, jtl, 0mp
Approved by:	jtl (mentor), bz (mentor)
Differential Revision:	https://reviews.freebsd.org/D19861

share/man/man4/pflog.4

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 31, 2007
+.Dd April 9, 2019
 .Dt PFLOG 4
 .Os
 .Sh NAME
@@ -47,13 +47,18 @@ on the
 interface, or stored to disk using
 .Xr pflogd 8 .
 .Pp
-The pflog0 interface is created automatically at boot if both
+The pflog0 interface is created when the
+.Nm
+module is loaded;
+further instances can be created using
+.Xr ifconfig 8 .
+The
+.Nm
+module is loaded automatically if both
 .Xr pf 4
 and
 .Xr pflogd 8
-are enabled;
-further instances can be created using
-.Xr ifconfig 8 .
+are enabled.
 .Pp
 Each packet retrieved on this interface has a header associated
 with it of length
@@ -87,6 +92,8 @@ Create a
 interface
 and monitor all packets logged on it:
 .Bd -literal -offset indent
+# ifconfig pflog create
+pflog1
 # ifconfig pflog1 up
 # tcpdump -n -e -ttt -i pflog1
 .Ed