[3.9] gh-87597: Document TimeoutExpired.stdout & .stderr types (GH-97685) (GH-97688)

This documents the behavior that has always been the case since timeout
support was introduced in Python 3.3.
(cherry picked from commit b05dd796)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Doc/library/subprocess.rst

@@ -195,7 +195,10 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
     .. attribute:: output
 
         Output of the child process if it was captured by :func:`run` or
-        :func:`check_output`.  Otherwise, ``None``.
+        :func:`check_output`.  Otherwise, ``None``.  This is always
+        :class:`bytes` when any output was captured regardless of the
+        ``text=True`` setting.  It may remain ``None`` instead of ``b''``
+        when no output was observed.
 
     .. attribute:: stdout
 
@@ -204,7 +207,9 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
     .. attribute:: stderr
 
         Stderr output of the child process if it was captured by :func:`run`.
-        Otherwise, ``None``.
+        Otherwise, ``None``.  This is always :class:`bytes` when stderr output
+        was captured regardless of the ``text=True`` setting.  It may remain
+        ``None`` instead of ``b''`` when no stderr output was observed.
 
     .. versionadded:: 3.3