Update description of Index._values/values/ndarray_values
closes #23504

@jbrockmendel @TomAugspurger 
Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/jorisvandenbossche/pandas/blob/8e14743a350adf3466d37a9c76df37e8b2f502ef/pandas/core/indexes/base.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23507?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@bd98841`). [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/23507/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23507?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #23507   +/-   ##
=========================================
  Coverage          ?   92.23%           
=========================================
  Files             ?      161           
  Lines             ?    51197           
  Branches          ?        0           
=========================================
  Hits              ?    47220           
  Misses            ?     3977           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (?)` | |
| #single | `42.27% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23507?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23507/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23507?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/23507?src=pr&el=footer). Last update [bd98841...36543b9](https://codecov.io/gh/pandas-dev/pandas/pull/23507?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
