Listing the shares for a document

I created a Google Docs document and shared it with other people.

The interface for Google Docs shows the shared people in the Folders/Sharing column.

Is there an api call in Zend_Gdata that I can use to find out who the shares are for a document?

I've looked through the authors, but I'm the only one listed.  I thought the rights may contain what I'm looking for, but I get null data from getRights().

Thanks,
Mark