Rephrase some of the media galleries permission information.

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

 <div id="pageData-name" class="pageData">Media Galleries</div>
 
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 <p>
 The media galleries API allows you to access media files (images,
 video, audio) from the user's local disks (with the user's consent).
 </p>
 
 <p>
 Using the API, you can prompt the user for permission to access the media
 galleries. The permission dialog will contain common media locations for
 the platform and will allow the user to add additional locations. From those
 locations, only media files will be present in the file system objects.
 </P>
 
 <h2 id="manifest">Manifest</h2>
-<p>You must declare the type of access to the media gallery you require
-in your extension's manifest to use the API.  Currently only the
-"mediaGalleriesRead" permission is allowed.  This permission does not
-currently trigger an install time permission prompt because it requires
-the user to grant permission at runtime.</p>
+<p>The media galleries API has two axes of permissions; the locations that
+can be accessed, and the type of access (read-only, read-write, add-files).</p>
 
-<p>
-You may also specify the "mediaGalleriesAllGalleries" permission to get
-access to all auto-detected media galleries on the user's computer.  This
+<p>On the location axis, specifying no location-type permission means that
+no media galleries are accessible until the user grants permission to
+specific media galleries at runtime using the media gallery configuration
+dialog. This dialog can be programmatically triggered.  Alternatively,
+specifying the "mediaGalleriesAllGalleries" permission grants access to all
+auto-detected media galleries on the user's computer. However, this
 permission displays an install time prompt indicating that the extension
-will have access to all of the user's media files.
-</p>
+will have access to all of the user's media files.</p>
+
+<p>On the access type axis, the "mediaGalleriesRead" permission grants the
+extension the right to read files. This permission does not trigger an install
+time permission prompt because the user must still grant access to particular
+galleries, either with the "mediaGalleriesAllGalleries" permission or at
+runtime by using the media gallery management dialog. For example:</p>
+
+<pre>{
+  "name": "My extension",
+  ...
+  "permissions": [
+    <b>"mediaGalleriesAllGalleries"</b>
+    <b>"mediaGalleriesRead"</b>
+  ],
+  ...
+}</pre>
+
+<p>This set of permissions will trigger an install time permission prompt
+and let the extension read from all auto-detected media galleries on the
+user's computer. The user may add or remove galleries using the
+media gallery management dialog, after which the extension will be able
+to read all the media files from galleries that the user has selected.</p>
+
+<p>Currently "mediaGalleriesRead" is the only access type supported by
+this API. Read-write and add-file access with be implemented soon.</p>
 
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