Fix the API name in the doc example, improve test.

This also makes the example more realistic, people are unlikely to want
this API to get a value into a simple `int` type, we've got direct APIs
for that. So have the example use an array as if it is a bignum of its
own.

I start by fixing the API name in the doc example as scoder@ noted in
the original code review after merge.  But I noticed one thing in the
test that could be done better so I added that as well: we need to
guarantee that all bytes of the result are overwritten.  This now
pre-fills the result with data in order to ensure that.  _(assuming
ctypes isn't undoing that behind the scenes...)_

Doc/c-api/long.rst

@@ -358,14 +358,14 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
 
    Copy the Python integer value to a native *buffer* of size *n_bytes*::
 
-      int value;
-      Py_ssize_t bytes = PyLong_CopyBits(v, &value, sizeof(value), -1);
+      unsigned bignum[4];  // Example size chosen by random die roll.
+      Py_ssize_t bytes = PyLong_AsNativeBits(v, &bignum, sizeof(bignum), -1);
       if (bytes < 0) {
-          // Error occurred
+          // A Python exception was set with the reason.
           return NULL;
       }
-      else if (bytes > sizeof(value)) {
-          // Overflow occurred, but 'value' contains as much as could fit
+      else if (bytes > sizeof(bignum)) {
+          // Overflow occurred, but 'bignum' contains as much as could fit.
       }
 
    *endianness* may be passed ``-1`` for the native endian that CPython was

Lib/test/test_capi/test_long.py

@@ -510,7 +510,9 @@ def test_long_asnativebytes(self):
         ]:
             with self.subTest(f"{v:X}-{len(expect_be)}bytes"):
                 n = len(expect_be)
-                buffer = bytearray(n)
+                # Fill the buffer with dummy data to ensure all bytes
+                # are overwritten.
+                buffer = bytearray(b'\xa5'*n)
                 expect_le = expect_be[::-1]
 
                 self.assertEqual(expect_n, asnativebytes(v, buffer, n, 0),