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.
---
I assume this needs neither a news entry nor bpo link.diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index 77f123c..96d77c4 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -978,7 +978,7 @@
constructor.
Setting encoding to ``NULL`` causes the default encoding to be used
-which is ASCII. The file system calls should use
+which is UTF-8. The file system calls should use
:c:func:`PyUnicode_FSConverter` for encoding file names. This uses the
variable :c:data:`Py_FileSystemDefaultEncoding` internally. This
variable should be treated as read-only: on some systems, it will be a