Document func parameter of locale.atof (GH-18183)

The second parameter (named `func`) has been present since the `locale`
module was introduced in eef1d4e8b1, but has never been documented.

This commit updates the documentation for `locale.atof` to clarify the
behavior of the function and how the `func` parameter is used.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>