Add the new debug option to the docs

All debug options are supposed to be listed in the "Running and
Debugging" section of the docs.

docs/reference/gtk/running.sgml

@@ -138,6 +138,10 @@ additional environment variables.
   this variable can be set to a list of debug options, which cause GTK+
   to print out different types of debugging information.
   <variablelist>
+    <varlistentry>
+      <term>actions</term>
+      <listitem><para>Actions and menu models.</para></listitem>
+    </varlistentry>
     <varlistentry>
       <term>baselines</term>
       <listitem><para>Baselines.</para></listitem>