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..e04476e7993de4d37c0103c6a118410ce7a227fe 100644
--- a/chrome/common/extensions/docs/apps/mediaGalleries.html
+++ b/chrome/common/extensions/docs/apps/mediaGalleries.html
@@ -200,10 +200,12 @@ 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>
+in your extension's manifest to use the API.  Currently the only access
+type permission is "mediaGalleriesRead," which will allow your extension
+to read from the media galleries.  This permission does not trigger an
+install time permission prompt because the user must still grant access
+to particular galleries at runtime unless the "mediaGalleriesAllGalleries"
+permission is used.</p>
 <p>
 You may also specify the "mediaGalleriesAllGalleries" permission to get
 access to all auto-detected media galleries on the user's computer.  This