[Extensions Docs] Update browser/page action documentation

chrome/common/extensions/docs/templates/intros/pageAction.html

 <p>
 Some examples:
 </p>
 <ul>
   <li> Subscribe to this page's RSS feed </li>
   <li> Make a slideshow out of this page's photos </li>
 </ul>
 
 <p>
 The RSS icon in the following screenshot
@@ skipping 27 matching lines @@
 <a href="manifest">extension manifest</a>
 like this:
 </p>
 
 <pre data-filename="manifest.json">
 {
   "name": "My extension",
   ...
   <b>"page_action": {
     "default_icon": {                    <em>// optional</em>
-      "19": "images/icon19.png",           <em>// optional</em>
-      "38": "images/icon38.png"            <em>// optional</em>
+      "16": "images/icon16.png",           <em>// optional</em>
+      "24": "images/icon24.png",           <em>// optional</em>
+      "32": "images/icon32.png"            <em>// optional</em>
     },
     "default_title": "Google Mail",      <em>// optional; shown in tooltip</em>
     "default_popup": "popup.html"        <em>// optional</em>
   }</b>,
   ...
 }</pre>
 
 <p>
-If you only provide one of the 19px or 38px icon size, the extension system will
-scale the icon you provide to the pixel density of the user's display, which
-can lose detail or make it look fuzzy. The old syntax for registering the
-default icon is still supported:
+You can provide any size icon to be used in Chrome, and Chrome will select the
+closest one and scale it to the appropriate size to fill the 16-dip space.
+However, if the exact size isn't provided, this scaling can cause the icon to
+lose detail or look fuzzy.
+</p>
+
+<p>
+Since devices with less-common scale factors like 1.5x or 1.2x are becoming more
+common, you are encouraged to provide multiple sizes for your icons. This also
+ensures that if the icon display size is ever changed, you don't need to do any
+more work to provide different icons!
+</p>
+
+<p>
+The old syntax for registering the default icon is still supported:
 </p>
 
 <pre data-filename="manifest.json">
 {
   "name": "My extension",
   ...
   <b>"page_action": {
     ...
-    "default_icon": "images/icon38.png"  <em>// optional</em>
-    <em>// equivalent to "default_icon": { "38": "images/icon38.png" }</em>
+    "default_icon": "images/icon32.png"  <em>// optional</em>
+    <em>// equivalent to "default_icon": { "32": "images/icon32.png" }</em>
   }</b>,
   ...
-}</pre>
+}
+</pre>
 
 <h2 id="ui">Parts of the UI</h2>
 
 <p>
 Like browser actions,
 page actions can have an icon,
 a tooltip, and popup;
 they can't have badges, however.
 In addition, page actions can be grayed out.
 You can find information about icons, tooltips, and popups
@@ skipping 34 matching lines @@
 
 <h2 id="examples"> Examples </h2>
 
 <p>
 You can find simple examples of using page actions in the
 <a href="https://chromium.googlesource.com/chromium/src/+/master/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">Samples</a>.
 </p>