docs: Add the missed usage type 'iscsi'

Pushed under trivial rule.

docs/formatsecret.html.in

@@ -41,8 +41,8 @@
       <dd>
         Specifies what this secret is used for.  A mandatory
         <code>type</code> attribute specifies the usage category, currently
-        only <code>volume</code> and <code>ceph</code> are defined.
-        Specific usage categories are described below.
+        only <code>volume</code>, <code>ceph</code> and <code>iscsi</code>
+        are defined. Specific usage categories are described below.
       </dd>
     </dl>