Add note in docs for interpolate that Only `method=linear` interpolation is supported on MultiIndexes.
Trying to interpolate a DataFrame with MultiIndex:

```
lib/python2.7/site-packages/pandas/core/generic.pyc in interpolate(self, method, axis, limit, inplace, downcast, **kwargs)
   2868 
   2869         if isinstance(self.index, MultiIndex) and method != 'linear':
-> 2870             raise ValueError("Only `method=linear` interpolation is supported "
   2871                              "on MultiIndexes.")
   2872 

ValueError: Only `method=linear` interpolation is supported on MultiIndexes.
```

add a note [in the docs](http://pandas.pydata.org/pandas-docs/dev/generated/pandas.Series.interpolate.html).
