#9110: update to ContextDecorator doc.

Doc/library/contextlib.rst

@@ -57,6 +57,7 @@ Functions provided:
    .. versionchanged:: 3.2
       Use of :class:`ContextDecorator`.
 
+
 .. function:: closing(thing)
 
    Return a context manager that closes *thing* upon completion of the block.  This
@@ -92,7 +93,10 @@ Functions provided:
    ``__enter__`` and ``__exit__`` as normal. ``__exit__`` retains its optional
    exception handling even when used as a decorator.
 
-   Example::
+   ``ContextDecorator`` is used by :func:`contextmanager`, so you get this
+   functionality automatically.
+
+   Example of ``ContextDecorator``::
 
       from contextlib import ContextDecorator
 
@@ -121,6 +125,21 @@ Functions provided:
       The bit in the middle
       Finishing
 
+   This change is just syntactic sugar for any construct of the following form::
+
+      def f():
+          with cm():
+              # Do stuff
+
+   ``ContextDecorator`` lets you instead write::
+
+      @cm()
+      def f():
+          # Do stuff
+
+   It makes it clear that the ``cm`` applies to the whole function, rather than
+   just a piece of it (and saving an indentation level is nice, too).
+
    Existing context managers that already have a base class can be extended by
    using ``ContextDecorator`` as a mixin class::