Update ipset doc

git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@8705 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb

docs/ipsets.xml

@@ -76,6 +76,20 @@
 
     <para>Example: "+Mirrors"</para>
 
+    <para>When using Shorewall-perl, the names of ipsets are restricted as
+    follows:</para>
+
+    <itemizedlist>
+      <listitem>
+        <para>They must begin with a letter (after the '+').</para>
+      </listitem>
+
+      <listitem>
+        <para>They must be composed of letters, digits or underscores
+        ("_").</para>
+      </listitem>
+    </itemizedlist>
+
     <para>The name of the set may optionally followed by:</para>
 
     <orderedlist numeration="loweralpha">