Update media galleries documentation.

chrome/common/extensions/api/media_galleries.idl

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // File-level comment to appease parser. Eventually this will not be necessary.
 
 namespace mediaGalleries {
 
   [inline_doc] enum GetMediaFileSystemsInteractivity {
     // Do not act interactively.
     no,
     // Ask the user to manage permitted media galleries.
     yes,
     // Ask the user to manage permitted galleries only if the return set would
     // otherwise be empty.
     if_needed
   };
 
   [inline_doc] dictionary MediaFileSystemsDetails {
-    // Whether to prompt the user for additional media galleries before
-    // returning the permitted set. Default is silent.
+    // Whether to prompt the user for permission to additional media galleries
+    // before returning the permitted set. Default is silent.  If the value
+    // 'yes' is passed, or if the application has not been granted access to
+    // any media galleries and the value 'if_needed' is passed, then the
+    // media gallery configuration dialog will be displayed.
     GetMediaFileSystemsInteractivity? interactive;
   };
 
   callback MediaFileSystemsCallback =
       void ([instanceOf=LocalFileSystem] optional object[] mediaFileSystems);
 
   interface Functions {
     // Get the media galleries configured in this user agent. If none are
     // configured or available, the callback will receive an empty array.
     static void getMediaFileSystems(optional MediaFileSystemsDetails details,
                                     MediaFileSystemsCallback callback);
   };
 
 };