DOC: Do no use 'type' as first word when specifying a return type (#21622)
Pycharm is otherwise confused and expects objects of type
'type' to be returned.

- [x] closes #21622 
- [x] tests added / passed (no code changes)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (not needed I guess)

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21623?src=pr&el=h1) Report
> Merging [#21623](https://codecov.io/gh/pandas-dev/pandas/pull/21623?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4922c0e0ab7044b8ed85299800e2efaf88035500?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/21623/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21623?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21623   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49547    49547           
=======================================
  Hits        45537    45537           
  Misses       4010     4010
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21623?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.23% <ø> (ø)` | :arrow_up: |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `92.66% <ø> (ø)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.2% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.22% <ø> (ø)` | :arrow_up: |
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.2% <ø> (ø)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.28% <ø> (ø)` | :arrow_up: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/21623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.43% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21623?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/21623?src=pr&el=footer). Last update [4922c0e...452cd99](https://codecov.io/gh/pandas-dev/pandas/pull/21623?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @aberres 