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.
validate_docstring.py results:
```
$ scripts/validate_docstrings.py  pandas.Series.interpolate
[...]
################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.Series.interpolate" correct. :)
``` 

I built the documentation HTML and the docstring looked as expected. Makes sense. Ping on green @WillAyd have you seen this failure in the CI on other PRs?

```
=================================== FAILURES ===================================
_______________ TestDatetimeIndex.test_stringified_slice_with_tz _______________
[gw0] darwin -- Python 3.5.6 /Users/vsts/miniconda3/envs/pandas-dev/bin/python

self = <pandas.tests.indexes.datetimes.test_datetime.TestDatetimeIndex object at 0x13accba58>

    def test_stringified_slice_with_tz(self):
        # GH#2658
        import datetime
        start = datetime.datetime.now()
>       idx = date_range(start=start, freq="1d", periods=10, tz='US/Eastern')

pandas/tests/indexes/datetimes/test_datetime.py:105: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/core/indexes/datetimes.py:1551: in date_range
    closed=closed, **kwargs)
pandas/core/arrays/datetimes.py:454: in _generate_range
    tz, ambiguous=ambiguous, nonexistent=nonexistent)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   raise pytz.NonExistentTimeError(stamp)
E   pytz.exceptions.NonExistentTimeError: 2019-03-10 02:01:53.911138

pandas/_libs/tslibs/conversion.pyx:1100: NonExistentTimeError
``` Hmm no that's a new one @TomAugspurger judging by the test I think it's unstable given the upcoming DST transition. I'll open a separate issue If I'm reading the test results correctly, the test failure is from line 105 here:

https://github.com/pandas-dev/pandas/blob/0a61ecdf6b4ea61a67afb4e3862df79adc07053a/pandas/tests/indexes/datetimes/test_datetime.py#L101-L108

That particular line was changed 5 days ago in #25263.

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=h1) Report
> Merging [#25491](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/011f0a6ebacb13619bf8225b3bc2370872b85769?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25491/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25491   +/-   ##
=======================================
  Coverage   41.72%   41.72%           
=======================================
  Files         173      173           
  Lines       52955    52955           
=======================================
  Hits        22093    22093           
  Misses      30862    30862
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `37.92% <ø> (ø)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `44.7% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `17.79% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `34.79% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `55.72% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `45.62% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=footer). Last update [011f0a6...a4152ae](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=h1) Report
> Merging [#25491](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/011f0a6ebacb13619bf8225b3bc2370872b85769?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25491/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25491   +/-   ##
=======================================
  Coverage   91.75%   91.75%           
=======================================
  Files         173      173           
  Lines       52955    52955           
=======================================
  Hits        48589    48589           
  Misses       4366     4366
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (ø)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `94.16% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=footer). Last update [011f0a6...a4152ae](https://codecov.io/gh/pandas-dev/pandas/pull/25491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebased after #25496 was merged. 

@WillAyd, CI is now green and this looks ready to merge.  Thanks @nmusolino !