Update core-icon API docs

Related to #19.

core-icon.html

@@ -8,7 +8,7 @@
 -->
 <!--
 
-The `core-icon` element displays an icon. By default an icon renders as 24px square.
+The `core-icon` element displays an icon. By default an icon renders as a 24px square.
 
 Example using src:
 
@@ -25,18 +25,35 @@
       }
     </style>
 
-Example using icon from default iconset:
+The core elements include several sets of icons. 
+To use the default set of icons, import  `core-icons.html` and use the `icon` attribute to specify an icon:
+
+Example of using an icon from the default iconset:
+    
+    &lt;!-- import default iconset and core-icon --&gt;
+    <link rel="import" href="/components/core-icons/core-icons.html">
 
     <core-icon icon="menu"></core-icon>
+    
+To use a different built-in set of icons, import  `core-icons/<iconset>-icons.html`, and
+specify the icon as `<iconset>:<icon>`. For example:
+
+    &lt;!-- import communication iconset and core-icon --&gt;
+    <link rel="import" href="/components/core-icons/communication-icons.html">
+
+    <core-icon icon="communication:email"></core-icon>
+    
+You can also create custom icon sets of bitmap or SVG icons. 
+
 
-Example using icon `cherry` from custom iconset `fruit`:
+Example of using an icon named `cherry` from a custom iconset with the ID `fruit`:
 
     <core-icon icon="fruit:cherry"></core-icon>
 
 See [core-iconset](#core-iconset) and [core-iconset-svg](#core-iconset-svg) for more information about
-how to use a custom iconset.
+how to create a custom iconset.
 
-See [core-icons](http://www.polymer-project.org/components/core-icons/demo.html) for the default set of icons. To use the default set of icons you'll need to include an import for `core-icons.html`. To use a different built-in set of icons, you'll need to include an import for `core-icons/iconsets/<iconset>.html`.
+See [core-icons](http://www.polymer-project.org/components/core-icons/demo.html) for the default set of icons.
 
 @group Polymer Core Elements
 @element core-icon