Rephrase some of the media galleries permission information.

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

Index: chrome/common/extensions/docs/apps/mediaGalleries.html
diff --git a/chrome/common/extensions/docs/apps/mediaGalleries.html b/chrome/common/extensions/docs/apps/mediaGalleries.html
index f1a03ecbf7e462e22a2f1cf829d9517c3e2c5a3a..dba3c66bb50469f30e2f36bc6da42c495d6fd289 100644
--- a/chrome/common/extensions/docs/apps/mediaGalleries.html
+++ b/chrome/common/extensions/docs/apps/mediaGalleries.html
@@ -199,17 +199,37 @@ 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>
-You may also specify the "mediaGalleriesAllGalleries" permission to get
-access to all auto-detected media galleries on the user's computer.  This
+<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>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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