gh-85045: clarified that the underlying buffer of a TextIOBase can be a RawIOBase (GH-134372)

Added a clarification that the underlying binary buffer of a TextIOBase can be a BufferedIOBase OR a RawIOBase
(cherry picked from commit 36eb711)

Co-authored-by: Alex Kautz <[email protected]>

Author: AlexKautz

Doc/library/io.rst

@@ -879,9 +879,10 @@ Text I/O
 
    .. attribute:: buffer
 
-      The underlying binary buffer (a :class:`BufferedIOBase` instance) that
-      :class:`TextIOBase` deals with.  This is not part of the
-      :class:`TextIOBase` API and may not exist in some implementations.
+      The underlying binary buffer (a :class:`BufferedIOBase`
+      or :class:`RawIOBase` instance) that :class:`TextIOBase` deals with.
+      This is not part of the :class:`TextIOBase` API and may not exist
+      in some implementations.
 
    .. method:: detach()