Merge pull request #3623 from dainiusjocas/master

Mention that there can be only a single level of leaf groups under the top group

en/reference/services-content.html

@@ -500,6 +500,7 @@ <h2 id="group">group</h2>
 Defines the <a href="../elasticity.html#grouped-distribution">hierarchical structure</a> of the cluster.
 Can not be used in conjunction with the <a href="#nodes">nodes</a> element.
 Groups can contain other groups or nodes, but not both.
+There can only be a single level of leaf groups under the top group.
 </p><p>
 When using groups in <a href="https://vespa.ai/">Open Source Vespa</a>,
 <a href="#searchable-copies">searchable-copies</a> and