Update page_action and browser_action docs to say that default_icon is optional not required.

chrome/common/extensions/docs/static/pageAction.html

 <div id="pageData-name" class="pageData">Page Actions</div>
 
 <!-- BEGIN AUTHORED CONTENT -->
 <p>
 Use page actions to put icons inside the address bar.
 Page actions represent actions
 that can be taken on the current page,
 but that aren't applicable to all pages.
 Some examples:
 </p>
@@ skipping 28 matching lines @@
 <p>
 Register your page action in the
 <a href="manifest.html">extension manifest</a>
 like this:
 </p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"page_action": {
-    "default_icon": "icons/foo.png", <em>// <b>required</b></em>
+    "default_icon": "icons/foo.png", <em>// optional</em>
     "default_title": "Do action",    <em>// optional; shown in tooltip</em>
     "default_popup": "popup.html"    <em>// optional</em>
   }</b>,
   ...
 }</pre>
 
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
 Like browser actions,
-page actions have an icon and
-can also have a tooltip and popup;
+page actions can have an icon,
+a tooltip, and popup;
 they can't have badges, however.
 In addition, page actions can appear and disappear.
 You can find information about icons, tooltips, and popups
 by reading about the
 <a href="browserAction.html#ui">browser action UI</a>.
 </p>
 
 <p>
 You make a page action appear and disappear using the
 <a href="#method-show">show()</a> and
@@ skipping 41 matching lines @@
 
 <p>
 You can find simple examples of using page actions in the
 <a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/pageAction/">examples/api/pageAction</a>
 directory.
 For other examples and for help in viewing the source code, see
 <a href="samples.html">Samples</a>.
 </p>
 
 <!-- END AUTHORED CONTENT -->