DOC: Reword Series.interpolate docstring for clarity
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The description of one group of methods in the `Series.interpolate` docstring currently reads:

> ‘nearest’, ‘zero’, ‘slinear’, ‘quadratic’, ‘cubic’, ‘spline’, ‘barycentric’, ‘polynomial’: Passed to scipy.interpolate.interp1d. Both ‘polynomial’ and ‘spline’ require that you also specify an order (int), e.g. df.interpolate(method='polynomial', order=4). These use the numerical values of the index.

In this item, it is unclear whether "These" in the final sentence refers to the 'polynomial' and 'spline' methods, or all eight methods in the group.  This ambiguity was the cause of confusion in issue #21662, for example.  This PR reorders the sentences for improved clarity.