Use the .warning selector for the distribution point

Author: Emmanuel Garcia

iron-component-page.html

@@ -24,6 +24,9 @@
 Loads Polymer element and behavior documentation using
 [Hydrolysis](https://github.com/PolymerLabs/hydrolysis) and renders a complete
 documentation page including demos (if available).
+
+To display a warning inside an `iron-component-page` element,
+add the `.warning` class to a child.
 -->
 
 <dom-module id="iron-component-page">
@@ -226,7 +229,7 @@
             <div id="catalog-heading" catalog-only>
               <h2><span>[[active]]</span> <span class="version" hidden$="[[!version]]">[[version]]</span></h2>
             </div>
-            <content></content>
+            <content select=".warning"></content>
             <iron-doc-viewer prefix="[[_fragmentPrefix]]" id="viewer" descriptor="{{_activeDescriptor}}"
               on-iron-doc-viewer-component-selected="_handleComponentSelectedEvent"></iron-doc-viewer>
             <div id="nodocs" hidden$="[[_activeDescriptor]]" class="no-docs">