DOC: Explicitly include "private" ExtensionArray methods in API docs
- [x] closes #24067
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

cc @jorisvandenbossche @TomAugspurger  @tadeja can you take a look at the built docs at http://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.api.extensions.ExtensionArray.html#pandas.api.extensions.ExtensionArray when the CI build on master for this finishes? Thanks! It looks good over here:
https://dev.pandas.io/reference/api/pandas.api.extensions.ExtensionArray.html#pandas.api.extensions.ExtensionArray
(Date: Jul 16, 2019 Version: 0.25.0rc0+76.g26bd34df2)

... while this one appears unchanged and I am just wondering if that's going to be OK:
http://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.api.extensions.ExtensionArray.html#pandas.api.extensions.ExtensionArray
(Date: Jun 18, 2019 Version: 0.25.0.dev0+752.g49f33f0d)
 I don't think the second one is being updated anymore.

On Tue, Jul 16, 2019 at 4:58 PM tadeja <notifications@github.com> wrote:

> Thanks! It looks good over here:
>
> https://dev.pandas.io/reference/api/pandas.api.extensions.ExtensionArray.html#pandas.api.extensions.ExtensionArray
> (Date: Jul 16, 2019 Version: 0.25.0rc0+76.g26bd34df2)
>
> ... while this one appears unchanged and I am just wondering if that's
> going to be OK:
>
> http://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.api.extensions.ExtensionArray.html#pandas.api.extensions.ExtensionArray
> (Date: Jun 18, 2019 Version: 0.25.0.dev0+752.g49f33f0d)
>
> —
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