Clarify that patch_stdout also redirect stderr (#2029)

Author: Carreau

src/prompt_toolkit/patch_stdout.py

@@ -40,7 +40,7 @@
 @contextmanager
 def patch_stdout(raw: bool = False) -> Generator[None, None, None]:
     """
-    Replace `sys.stdout` by an :class:`_StdoutProxy` instance.
+    Replace `sys.stdout` and `sys.stderr` by an :class:`_StdoutProxy` instance.
 
     Writing to this proxy will make sure that the text appears above the
     prompt, and that it doesn't destroy the output from the renderer.  If no
@@ -52,6 +52,9 @@ def patch_stdout(raw: bool = False) -> Generator[None, None, None]:
         is changed. Printing to stdout will be scheduled in the event loop
         that's active when the context manager is created.
 
+    Warning: In order for all text to appear above the prompt `stderr` will also
+        be redirected to the stdout proxy.
+
     :param raw: (`bool`) When True, vt100 terminal escape sequences are not
                 removed/escaped.
     """