DOC: Add experimental note to DatetimeArray and TimedeltaArray
DOC: Mention PeriodArray and IntervalArray

Closes #24870
is it worth putting a similar warning in whatsnew / docs-proper as well (maybe in timeseries.rst & timedelta.rst top-section)? Wasn't sure. Neither of those have sections for DatetimeArray or TimedletaArray specifically. The only place is in `api/arrays.rst`. @TomAugspurger yeah right. Maybe we should add that (certainly not necessary for now though) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=h1) Report
> Merging [#24882](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c588437f116a080cdcabb023a5e746dda2cb63c9?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/24882/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24882   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files         166      166           
  Lines       52413    52413           
=======================================
  Hits        48425    48425           
  Misses       3988     3988
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `42.89% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24882/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `88.3% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24882/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.79% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24882?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/24882?src=pr&el=footer). Last update [c588437...e068fee](https://codecov.io/gh/pandas-dev/pandas/pull/24882?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/24882?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@0a4665a`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24882/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #24882   +/-   ##
=========================================
  Coverage          ?   92.39%           
=========================================
  Files             ?      166           
  Lines             ?    52413           
  Branches          ?        0           
=========================================
  Hits              ?    48425           
  Misses            ?     3988           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (?)` | |
| #single | `42.89% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24882/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `88.3% <ø> (ø)` | |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24882/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.79% <ø> (ø)` | |
| [pandas/core/arrays/integer.py](https://codecov.io/gh/pandas-dev/pandas/pull/24882/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVnZXIucHk=) | `96.32% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24882?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/24882?src=pr&el=footer). Last update [0a4665a...5d69738](https://codecov.io/gh/pandas-dev/pandas/pull/24882?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I would add similar notices to IntegerArray, and to the whatsnew file (but can also be in separate PR) @jorisvandenbossche is 
https://github.com/pandas-dev/pandas/pull/24882/commits/e65fa9077ad3991965795499990e2d8bc77f1f2e what you had in mind for IntegerArray? > @jorisvandenbossche is
e65fa90 what you had in mind for IntegerArray?

yep! can you add the same in the integer_na.rst ? Done in 
https://github.com/pandas-dev/pandas/pull/24882/commits/5bec9e47a1ead6ad2ed07a4072b15a32a88e8186