Porters Handbook: add some clarifications for Poudreire:

- add a hint to create the distfiles directory if it does not already exist
- add the new-style syntax to use a custom ports tree

PR:		237794
Submitted by:	vladimir.chukharev@gmail.com
Event:		July 2020 Bugathon

en_US.ISO8859-1/books/porters-handbook/testing/chapter.xml

@@ -359,7 +359,9 @@ SVN_HOST=svn.FreeBSD.org</programlisting>
 	      example, <application>Poudriere</application> and the
 	      host share the distfiles storage directory.  This avoids
 	      downloading tarballs which are already present on the
-	      system.</para>
+	      system.  Please create this directory if it does not
+	      already exist so that <application>Poudriere</application>
+	      can find it.</para>
 	  </listitem>
 	</varlistentry>
 
@@ -604,7 +606,21 @@ subversive           svn+https  /poudriere/ports/subversive</screen>
 	<filename>/work/ports</filename>, point
 	<application>Poudriere</application> to the location:</para>
 
-      <screen>&prompt.root; <userinput>poudriere ports -c -F -f none -M /work/ports -p development</userinput></screen>
+      <itemizedlist>
+	<listitem>
+      	  <para>For <application>Poudriere</application> older than
+	    version 3.1.20:</para>
+
+	  <screen>&prompt.root; <userinput>poudriere ports -c -F -f none -M /work/ports -p development</userinput></screen>
+	</listitem>
+
+	<listitem>
+	  <para>For <application>Poudriere</application> version 3.1.20
+	    and later:</para>
+
+	  <screen>&prompt.root; <userinput>poudriere ports -c -m null -M /work/ports -p development</userinput></screen>
+	</listitem>
+      </itemizedlist>
 
       <para>This will be listed in the table of known trees:</para>