[3.14] gh-137368: document __index__() support for PyLong_AsInt32/64() (GH-137369) (#137380)

gh-137368: document __index__() support for PyLong_AsInt32/64() (GH-137369)
(cherry picked from commit 4dd85b3)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: miss-islington

Doc/c-api/long.rst

@@ -372,6 +372,10 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
    Set *\*value* to a signed C :c:expr:`int32_t` or :c:expr:`int64_t`
    representation of *obj*.
 
+   If *obj* is not an instance of :c:type:`PyLongObject`, first call its
+   :meth:`~object.__index__` method (if present) to convert it to a
+   :c:type:`PyLongObject`.
+
    If the *obj* value is out of range, raise an :exc:`OverflowError`.
 
    Set *\*value* and return ``0`` on success.