bpo-36760: Clarify subprocess capture_output docs. (GH-13322)

Clarify how to capture stdout and stderr combined into one stream.
(cherry picked from commit e883091)

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

Doc/library/subprocess.rst

@@ -55,7 +55,9 @@ compatibility with older versions, see the :ref:`call-function-trio` section.
    If *capture_output* is true, stdout and stderr will be captured.
    When used, the internal :class:`Popen` object is automatically created with
    ``stdout=PIPE`` and ``stderr=PIPE``. The *stdout* and *stderr* arguments may
-   not be supplied at the same time as *capture_output*.
+   not be supplied at the same time as *capture_output*.  If you wish to capture
+   and combine both streams into one, use ``stdout=PIPE`` and ``stderr=STDOUT``
+   instead of *capture_output*.
 
    The *timeout* argument is passed to :meth:`Popen.communicate`. If the timeout
    expires, the child process will be killed and waited for.  The