docs(python): Fix docstring mistake for polars.concat_str

[wsyxbcl]

The default behavior of ignore_nulls in concat_str is incorrectly documented.

Not sure which default value is more intuitive, I simply corrected the documentation to avoid breaking changes. Though it differs from Series.list.concat or Series.string.concat, where the default value is True, introducing some inconsistency.

[reswqa]

Though it differs from Series.list.concat or Series.string.concat, where the default value is True, introducing some inconsistency.

IIRC, this is because we don't want to make a breaking change before v1.0 when we introduce the ignore_nulls for concat_str.

[wsyxbcl]

Though it differs from Series.list.concat or Series.string.concat, where the default value is True, introducing some inconsistency.

IIRC, this is because we don't want to make a breaking change before v1.0 when we introduce the ignore_nulls for concat_str.

That's fine, we simply fix the doc then