Document cluster-scoped template variables for resource tags

- Add ${clustername} and ${clustertype} to the template variables list
- Note they resolve per host at apply time, not at deployment time
- Add example showing per-cluster cost tracking pattern

Author: gjoranv

en/reference/applications/deployment.html

@@ -386,18 +386,49 @@ <h2 id="resource-tags">resource-tags</h2>
 </p>
 
 <p id="resource-tags-template-variables"><strong>Template variables.</strong>
-Tag values may reference the following template variables, which are resolved at deployment time:</p>
-<ul>
-  <li><code>${tenant}</code></li>
-  <li><code>${application}</code></li>
-  <li><code>${instance}</code></li>
-  <li><code>${environment}</code></li>
-  <li><code>${region}</code></li>
-</ul>
-<p>
-  Resolved values are lowercased to match the label constraints above. Referencing an unknown
-  variable causes the deployment to fail.
-</p>
+Tag values may reference the following template variables.
+Resolved values are lowercased to match the label constraints above.
+Referencing an unknown variable causes the deployment to fail.
+Variables can be combined, e.g. <code>value="${environment}-${clustertype}"</code>.</p>
+<table class="table">
+  <thead>
+  <tr>
+    <th style="width:200px">Variable</th>
+    <th>Description</th>
+  </tr>
+  </thead>
+  <tbody>
+  <tr>
+    <td><code>${tenant}</code></td>
+    <td>The tenant name.</td>
+  </tr>
+  <tr>
+    <td><code>${application}</code></td>
+    <td>The application name.</td>
+  </tr>
+  <tr>
+    <td><code>${instance}</code></td>
+    <td>The instance name.</td>
+  </tr>
+  <tr>
+    <td><code>${environment}</code></td>
+    <td>The deployment environment, e.g. <code>prod</code>, <code>dev</code>.</td>
+  </tr>
+  <tr>
+    <td><code>${region}</code></td>
+    <td>The deployment region, e.g. <code>aws-us-east-1c</code>.</td>
+  </tr>
+  <tr>
+    <td><code>${clustername}</code></td>
+    <td>The cluster ID from <a href="services.html">services.xml</a>,
+      e.g. <code>default</code>, <code>music</code>.</td>
+  </tr>
+  <tr>
+    <td><code>${clustertype}</code></td>
+    <td>The Vespa cluster type: <code>container</code>, <code>content</code>, or <code>admin</code>.</td>
+  </tr>
+  </tbody>
+</table>
 
 <p><strong>Reconciliation.</strong>
 Tags are applied to virtual machines and attached disks. When tags are changed, added, or removed