[3.13] pythongh-143834: Fix PyLong_AsNativeBytes docs for negative number padding (pythonGH-143840) (pythonGH-143855)

(cherry picked from commit 421bd17)

Co-authored-by: Muneeb Ullah <moneebullah25@gmail.com>

Author: miss-islington

Doc/c-api/long.rst

@@ -403,8 +403,8 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
 
    Otherwise, returns the number of bytes required to store the value.
    If this is equal to or less than *n_bytes*, the entire value was copied.
-   All *n_bytes* of the buffer are written: large buffers are padded with
-   zeroes.
+   All *n_bytes* of the buffer are written: remaining bytes filled by
+   copies of the sign bit.
 
    If the returned value is greater than *n_bytes*, the value was
    truncated: as many of the lowest bits of the value as could fit are written,