DOC: Update the Series.str.len docstring
[X] PR title is "DOC: update the docstring"
[X] The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
[X] The html version looks good: python doc/make.py --single

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22187?src=pr&el=h1) Report
> Merging [#22187](https://codecov.io/gh/pandas-dev/pandas/pull/22187?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/35dd15ba150175e8522e2d443ee4118b0d5c57ce?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22187/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22187?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22187   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         169      169           
  Lines       50694    50694           
=======================================
  Hits        46671    46671           
  Misses       4023     4023
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <100%> (ø)` | :arrow_up: |
| #single | `42.32% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22187?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22187/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22187?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/22187?src=pr&el=footer). Last update [35dd15b...f395314](https://codecov.io/gh/pandas-dev/pandas/pull/22187?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @IsvenC 