Document func parameter of locale.atof

The second parameter (named `func`) has been present since the `locale`
module was introduced in eef1d4e, 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>

Author: kevinoid

Doc/library/locale.rst

@@ -423,10 +423,10 @@ The :mod:`locale` module defines the following exception and functions:
     .. versionadded:: 3.5
 
 
-.. function:: atof(string)
+.. function:: atof(string, func=float)
 
-   Converts a string to a floating point number, following the :const:`LC_NUMERIC`
-   settings.
+   Converts a string to a number, following the :const:`LC_NUMERIC` settings,
+   by calling *func* on the result of calling :func:`delocalize` on *string*.
 
 
 .. function:: atoi(string)