DOC: add IndexSlice docstring + add to api docs
Hello Developers,

I could not find any information about "Pandas.IndexSlice" function in API Reference.There is only one example  at this [link](http://pandas.pydata.org/pandas-docs/stable/advanced.html#advanced-mi-slicers) . I want to filter the pivot table  as in Excel.But i am confused to using between loc(slice(...)) and IndexSlice.Please could you show some practical easy methods  to filter the pivot table.

Thank you very much...
