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.
