Improve Documentation for pandas.Series.interpolate as to how index is used

From the docs:

> ‘linear’: ignore the index and treat the values as equally spaced. default
> ‘index’, ‘values’: use the actual numerical values of the index

but the remaining types, for example ‘quadratic’, ‘cubic’, ‘barycentric’, `krogh`, don't specify whether they assume equal spacing or use the index values. 

I believe that they DO use the index values.
