DOC/API: expose ExtensionArray in public api
cc @jorisvandenbossche  do you want to include these in the public API docs? It's nice to have a place to link to I think.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=h1) Report
> Merging [#20795](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/31e77b0b4129134d20cd5027dac0adb9fbd2dca0?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20795/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20795      +/-   ##
==========================================
- Coverage   91.85%   91.82%   -0.03%     
==========================================
  Files         153      153              
  Lines       49310    49319       +9     
==========================================
- Hits        45292    45286       -6     
- Misses       4018     4033      +15
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.89% <75%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `84.14% <ø> (ø)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `92.01% <100%> (-0.03%)` | :arrow_down: |
| [pandas/api/extensions/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL2FwaS9leHRlbnNpb25zL19faW5pdF9fLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (-0.21%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.63% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/20795/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.07% <0%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=footer). Last update [31e77b0...fc3dfaa](https://codecov.io/gh/pandas-dev/pandas/pull/20795?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!