Update media galleries documentation.

chrome/common/extensions/api/media_galleries.idl

Index: chrome/common/extensions/api/media_galleries.idl
diff --git a/chrome/common/extensions/api/media_galleries.idl b/chrome/common/extensions/api/media_galleries.idl
index d0779d2c772821d7d8dfb0959a5e9218623c2a85..70b6153d39d52264753babbb09b8ef91acbb30af 100644
--- a/chrome/common/extensions/api/media_galleries.idl
+++ b/chrome/common/extensions/api/media_galleries.idl
@@ -17,8 +17,11 @@ namespace mediaGalleries {
   };
 
   [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;
   };