gh-91156: Document how TextIOWrapper interacts with UTF-8 mode (GH-132885)

Document how TextIOWrapper interacts with UTF-8 mode
(cherry picked from commit 245cd6c)

Co-authored-by: Malcolm Smith <[email protected]>

Author: miss-islington

Doc/library/io.rst

@@ -950,7 +950,8 @@ Text I/O
    :class:`TextIOBase`.
 
    *encoding* gives the name of the encoding that the stream will be decoded or
-   encoded with.  It defaults to :func:`locale.getencoding`.
+   encoded with.  In :ref:`UTF-8 Mode <utf8-mode>`, this defaults to UTF-8.
+   Otherwise, it defaults to :func:`locale.getencoding`.
    ``encoding="locale"`` can be used to specify the current locale's encoding
    explicitly. See :ref:`io-text-encoding` for more information.