bpo-28516: document contextlib.ExitStack.__enter__ behavior (GH-31636)

The enter_context is updated with following information: 'The :meth:`__enter__` method 
      returns the ExitStack instance, and performs no additional operations.'

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Doc/library/contextlib.rst

@@ -502,6 +502,9 @@ Functions and classes provided:
           # the with statement, even if attempts to open files later
           # in the list raise an exception
 
+   The :meth:`__enter__` method returns the :class:`ExitStack` instance, and
+   performs no additional operations.
+
    Each instance maintains a stack of registered callbacks that are called in
    reverse order when the instance is closed (either explicitly or implicitly
    at the end of a :keyword:`with` statement). Note that callbacks are *not*