Add <usage> to <secret> docs

* docs/formatsecret.html.in, docs/formatsecret.html: Document <usage
  type='volume'>, replacing stand-alone <volume>.
* docs/schemas/secret.rng: Update schema to require <usage
  type='volume'>

docs/formatsecret.html

@@ -152,17 +152,30 @@
         An unique identifier for this secret (not necessarily in the UUID
         format).  If omitted when defining a new secret, a random UUID is
         generated.
-      </dd><dt><code>volume</code></dt><dd>Key of a volume this secret is associated with.  It is safe to delete
-        the secret after the volume is deleted.
       </dd><dt><code>description</code></dt><dd>A human-readable description of the purpose of the secret.
+      </dd><dt><code>usage</code></dt><dd>
+	Specifies what this secret is used for.  A mandatory
+	<code>type</code> attribute specifies the usage category, currently
+	only <code>volume</code> is defined.  Specific usage categories are
+	described below.
       </dd></dl>
+        <h3>Usage type "volume"</h3>
+        <p>
+      This secret is associated with a volume, and it is safe to delete the
+      secret after the volume is deleted.  The <code>&lt;usage
+      type='volume'&gt;</code> element must contain a
+      single <code>volume</code> element that specifies the key of the volume
+      this secret is associated with.
+    </p>
         <h2>
           <a name="example" id="example">Example</a>
         </h2>
         <pre>
       &lt;secret ephemeral='no' private='yes'&gt;
-         &lt;volume&gt;/var/lib/libvirt/images/mail.img&lt;/volume&gt;
          &lt;description&gt;LUKS passphrase for the main hard drive of our mail server&lt;/description&gt;
+         &lt;usage type='volume'&gt;
+            &lt;volume&gt;/var/lib/libvirt/images/mail.img&lt;/volume&gt;
+         &lt;/usage&gt;
       &lt;/secret&gt;</pre>
       </div>
     </div>

docs/formatsecret.html.in

@@ -32,21 +32,36 @@
         format).  If omitted when defining a new secret, a random UUID is
         generated.
       </dd>
-      <dt><code>volume</code></dt>
-      <dd>Key of a volume this secret is associated with.  It is safe to delete
-        the secret after the volume is deleted.
-      </dd>
       <dt><code>description</code></dt>
       <dd>A human-readable description of the purpose of the secret.
       </dd>
+      <dt><code>usage</code></dt>
+      <dd>
+	Specifies what this secret is used for.  A mandatory
+	<code>type</code> attribute specifies the usage category, currently
+	only <code>volume</code> is defined.  Specific usage categories are
+	described below.
+      </dd>
     </dl>
 
+    <h3>Usage type "volume"</h3>
+
+    <p>
+      This secret is associated with a volume, and it is safe to delete the
+      secret after the volume is deleted.  The <code>&lt;usage
+      type='volume'&gt;</code> element must contain a
+      single <code>volume</code> element that specifies the key of the volume
+      this secret is associated with.
+    </p>
+
     <h2><a name="example">Example</a></h2>
 
     <pre>
       &lt;secret ephemeral='no' private='yes'&gt;
-         &lt;volume&gt;/var/lib/libvirt/images/mail.img&lt;/volume&gt;
          &lt;description&gt;LUKS passphrase for the main hard drive of our mail server&lt;/description&gt;
+         &lt;usage type='volume'&gt;
+            &lt;volume&gt;/var/lib/libvirt/images/mail.img&lt;/volume&gt;
+         &lt;/usage&gt;
       &lt;/secret&gt;</pre>
   </body>
 </html>

docs/schemas/secret.rng

@@ -34,11 +34,23 @@
 	  </element>
 	</optional>
 	<optional>
-	  <element name='volume'>
+	  <element name='usage'>
+	    <choice>
+	      <ref name='usagevolume'>
+	    </choice>
 	    <text/>
 	  </element>
 	</optional>
       </interleave>
     </element>
   </define>
+
+  <define name='usagevolume'>
+    <attribute name='type'>
+      <value>volume</value>
+    </attribute>
+    <element name='volume'>
+      <text/>
+    </element>
+  </define>
 </grammar>