DOC: Improved the docstring of Series.str.findall
Docstring assigned to PyData Mallorca in Pandas Documentation Sprint (https://python-sprints.github.io/pandas/).

Please, any indication to improve our PR will be appreciated!

Happy to contribute :-)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19982?src=pr&el=h1) Report
> Merging [#19982](https://codecov.io/gh/pandas-dev/pandas/pull/19982?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/63ce781c9ebfb7d308adb3be62755e2bdf6c0d8e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19982/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19982?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19982      +/-   ##
==========================================
+ Coverage   91.71%   91.72%   +<.01%     
==========================================
  Files         150      150              
  Lines       49112    49112              
==========================================
+ Hits        45045    45047       +2     
+ Misses       4067     4065       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19982?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/19982/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19982/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.85% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19982?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/19982?src=pr&el=footer). Last update [63ce781...cd7223b](https://codecov.io/gh/pandas-dev/pandas/pull/19982?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche all good? (i'll merge in a few after an xfail PR) All comments solved. Thanks! Thanks!