Extend descriptions and comments about the need to create /etc/pf.conf.

FreeBSD removed the default /etc/pf.conf file in previous releases, but
the documentation kept mentioning it like any other file present in the
system.  Change pf.conf(5) to mention in the description of the default
ruleset location that this file needs to be created manually. Also, the
default rc.conf file had it's comment extended a bit to let people know
that this file does not exist by default.

PR:		    231977
Submitted by:	    koobs@
Reviewed by:	    kp@, 0mp@
Approved by:	    kp@
MFC after:	    10 days
Differential Revision:	https://reviews.freebsd.org/D19530

libexec/rc/rc.conf

@@ -208,7 +208,8 @@ ipfs_enable="NO"		# Set to YES to enable saving and restoring
 ipfs_program="/sbin/ipfs"	# where the ipfs program lives
 ipfs_flags=""			# additional flags for ipfs
 pf_enable="NO"			# Set to YES to enable packet filter (pf)
-pf_rules="/etc/pf.conf"		# rules definition file for pf
+pf_rules="/etc/pf.conf"		# rules definition file for pf (nonexistent
+				# by default)
 pf_program="/sbin/pfctl"	# where the pfctl program lives
 pf_flags=""			# additional flags for pfctl
 pflog_enable="NO"		# Set to YES to enable packet filter logging

share/man/man5/pf.conf.5

@@ -28,7 +28,7 @@
 .\" ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 5, 2019
+.Dd March 10, 2019
 .Dt PF.CONF 5
 .Os
 .Sh NAME
@@ -3053,6 +3053,8 @@ include        = "include" filename
 Host name database.
 .It Pa /etc/pf.conf
 Default location of the ruleset file.
+The file has to be created manually as it is not installed with a
+standard installation.
 .It Pa /etc/pf.os
 Default location of OS fingerprints.
 .It Pa /etc/protocols