bpo-44522: Fix inaccurate information in open() function (GH-27650)

- Use "Low surrogate code units" instead of "Unicode Private Use Area"

Author: dukecat0

Doc/library/functions.rst

@@ -1208,9 +1208,9 @@ are always available.  They are listed here in alphabetical order.
    * ``'replace'`` causes a replacement marker (such as ``'?'``) to be inserted
      where there is malformed data.
 
-   * ``'surrogateescape'`` will represent any incorrect bytes as code
-     points in the Unicode Private Use Area ranging from U+DC80 to
-     U+DCFF.  These private code points will then be turned back into
+   * ``'surrogateescape'`` will represent any incorrect bytes as low
+     surrogate code units ranging from U+DC80 to U+DCFF.
+     These surrogate code units will then be turned back into
      the same bytes when the ``surrogateescape`` error handler is used
      when writing data.  This is useful for processing files in an
      unknown encoding.