DOC: Adding missing module, causing sphinx not to find functions/methods
sphinx is generating warnings because we reference API pages that don't exist.

This is because autosummary is processed before the `currentmodule` directive is injected to `{{ header }}`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=h1) Report
> Merging [#24420](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/159772d0c9d49977d4b7f871c62c26e06890143a?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/24420/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24420   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         163      163           
  Lines       51950    51950           
=======================================
  Hits        47953    47953           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24420?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/24420?src=pr&el=footer). Last update [159772d...9160722](https://codecov.io/gh/pandas-dev/pandas/pull/24420?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/24420?src=pr&el=h1) Report
> Merging [#24420](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/159772d0c9d49977d4b7f871c62c26e06890143a?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/24420/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24420   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         163      163           
  Lines       51950    51950           
=======================================
  Hits        47953    47953           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24420?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/24420?src=pr&el=footer). Last update [159772d...4d3f527](https://codecov.io/gh/pandas-dev/pandas/pull/24420?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Sorry, my fault, had a problem in my pandas set up, and I was getting sphinx warnings that weren't true, about those methods/functions not existing. Just couple of the were real, because of the module being missing. thanks @datapythonista 