[3.9] Document func parameter of locale.atof (GH-18183) (GH-32261)

[3.9] Document func parameter of locale.atof (GH-18183) (GH-32261)

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>.
(cherry picked from commit 208da6d508bb2683732151f4ae288dfc8001267c)

Co-authored-by: Kevin Locke <kevin@kevinlocke.name>