Add note about irkerhook and python 3 to docs

irkerhook.xml

@@ -174,6 +174,9 @@ are available as %() substitutions.</para>
 </varlistentry>
 </variablelist>
 
+<para>irkerhook.py will run under both python 2 and python 3, but it does
+not support mercurial repositories under python 3 yet.</para>
+
 <refsect2 id="git"><title>git</title>
 
 <para>Under git, the normal way to invoke this hook (from within the