DOC: State that we support scalars in to_numeric
We [support it](https://github.com/pandas-dev/pandas/blob/0c4113f/pandas/core/tools/numeric.py#L114-L120) and [test it](https://github.com/pandas-dev/pandas/blob/0c4113f/pandas/tests/tools/test_numeric.py#L174-L185) already.

xref #24910.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=h1) Report
> Merging [#24944](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0c4113fa0906273007cc12a4bcadff85d943dc84?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/24944/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24944   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52388    52388           
=======================================
  Hits        48396    48396           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/24944/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `100% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24944?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/24944?src=pr&el=footer). Last update [0c4113f...e3a38ba](https://codecov.io/gh/pandas-dev/pandas/pull/24944?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/24944?src=pr&el=h1) Report
> Merging [#24944](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0c4113fa0906273007cc12a4bcadff85d943dc84?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/24944/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24944   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52388    52388           
=======================================
  Hits        48396    48396           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/24944/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `100% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24944?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/24944?src=pr&el=footer). Last update [0c4113f...e3a38ba](https://codecov.io/gh/pandas-dev/pandas/pull/24944?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
