DOC: followup to #20583, observed kwarg for .groupby
closes #20902

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20941?src=pr&el=h1) Report
> Merging [#20941](https://codecov.io/gh/pandas-dev/pandas/pull/20941?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/28dbae9f306ade549eb1edd5484b3e1da758bcdb?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/20941/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20941?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20941   +/-   ##
=======================================
  Coverage   91.81%   91.81%           
=======================================
  Files         153      153           
  Lines       49479    49479           
=======================================
  Hits        45428    45428           
  Misses       4051     4051
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <100%> (ø)` | :arrow_up: |
| #single | `41.85% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20941?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20941/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.98% <ø> (ø)` | :arrow_up: |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20941/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `92.58% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20941?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/20941?src=pr&el=footer). Last update [28dbae9...4cf9153](https://codecov.io/gh/pandas-dev/pandas/pull/20941?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
