Add comments to explain different types of returned results

Hello @nicks, @dpup, 

Please review the following commits I made in branch 'xiao-result'.

6c097ef68ed498b6d4c5e6f9b4d612f3b354ad19 (2013-11-26 12:28:34 -0800)
Add comments to explain different types of returned results

Check list:


R=@nicks
R=@dpup