Correct the documented default encoding (GH-18429)

Correct the documented default encoding (GH-18429)

From the source for `PyUnicode_Decode`, the implementation is:
```
if (encoding == NULL) {
    return PyUnicode_DecodeUTF8Stateful(s, size, errors, NULL);
}
```
which is pretty clearly not defaulting to ASCII.

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I assume this needs neither a news entry nor bpo link.