gh-139452: Clarify redirect_stdout, stderr behavior

These don't "redirect" like a terminal where typically the file pointed
to by fd 1 and 2 are made the "same" by the preparing process (ex. by
`dup2` the given fd into that specific fd number). Rather, these context
managers set the respective globals to the provided value. In most cases
that should be a "file object".

Author: cmaloney

Doc/library/contextlib.rst

@@ -327,10 +327,12 @@ Functions and classes provided:
 .. function:: redirect_stdout(new_target)
 
    Context manager for temporarily redirecting :data:`sys.stdout` to
-   another file or file-like object.
+   another :term:`file object`.
 
    This tool adds flexibility to existing functions or classes whose output
-   is hardwired to stdout.
+   is hardwired to :data:`sys.stdout`. This does not modify underlying file
+   objects or file descriptors. It sets the global :data:`sys.stdout` to the
+   provided value and at context exit sets it to the previous value.
 
    For example, the output of :func:`help` normally is sent to *sys.stdout*.
    You can capture that output in a string by redirecting the output to an
@@ -366,8 +368,9 @@ Functions and classes provided:
 
 .. function:: redirect_stderr(new_target)
 
-   Similar to :func:`~contextlib.redirect_stdout` but redirecting
-   :data:`sys.stderr` to another file or file-like object.
+   Similar to :func:`~contextlib.redirect_stdout` but redirecting the global
+   :data:`sys.stderr` to another value, typically a :term:`file object`
+   returned from :func:`open`.
 
    This context manager is :ref:`reentrant <reentrant-cms>`.