Added full explanation for MENU item lists

I didn't find any explanation in the documentation for all three arguments in each list representing a menu item in the MENU helper, so I added it.  Credit for the explanation of the second argument goes to Anthony: http://stackoverflow.com/questions/10071468/adding-a-view-as-menu-item-using-menu-py-in-web2py
Merge pull request #205 from jegesh/master

Added full explanation for MENU item lists