bpo-32436: Document PEP 567 changes to asyncio. (GH-7073)

(cherry picked from commit 28b9178023a445b1da2694774c265cd4b7a244ec)

Co-authored-by: Yury Selivanov <yury@magic.io>
diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 2770fa6..9d7f236 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -124,7 +124,7 @@
    parameters in debug mode, whereas ``lambda`` functions have a poor
    representation.
 
-.. method:: AbstractEventLoop.call_soon(callback, \*args)
+.. method:: AbstractEventLoop.call_soon(callback, *args, context=None)
 
    Arrange for a callback to be called as soon as possible.  The callback is
    called after :meth:`call_soon` returns, when control returns to the event
@@ -137,19 +137,31 @@
    Any positional arguments after the callback will be passed to the
    callback when it is called.
 
+   An optional keyword-only *context* argument allows specifying a custom
+   :class:`contextvars.Context` for the *callback* to run in.  The current
+   context is used when no *context* is provided.
+
    An instance of :class:`asyncio.Handle` is returned, which can be
    used to cancel the callback.
 
    :ref:`Use functools.partial to pass keywords to the callback
    <asyncio-pass-keywords>`.
 
-.. method:: AbstractEventLoop.call_soon_threadsafe(callback, \*args)
+   .. versionchanged:: 3.7
+      The *context* keyword-only parameter was added. See :pep:`567`
+      for more details.
+
+.. method:: AbstractEventLoop.call_soon_threadsafe(callback, *args, context=None)
 
    Like :meth:`call_soon`, but thread safe.
 
    See the :ref:`concurrency and multithreading <asyncio-multithreading>`
    section of the documentation.
 
+   .. versionchanged:: 3.7
+      The *context* keyword-only parameter was added. See :pep:`567`
+      for more details.
+
 
 .. _asyncio-delayed-calls:
 
@@ -166,7 +178,7 @@
    Timeouts (relative *delay* or absolute *when*) should not exceed one day.
 
 
-.. method:: AbstractEventLoop.call_later(delay, callback, *args)
+.. method:: AbstractEventLoop.call_later(delay, callback, *args, context=None)
 
    Arrange for the *callback* to be called after the given *delay*
    seconds (either an int or float).
@@ -182,10 +194,18 @@
    is called. If you want the callback to be called with some named
    arguments, use a closure or :func:`functools.partial`.
 
+   An optional keyword-only *context* argument allows specifying a custom
+   :class:`contextvars.Context` for the *callback* to run in.  The current
+   context is used when no *context* is provided.
+
    :ref:`Use functools.partial to pass keywords to the callback
    <asyncio-pass-keywords>`.
 
-.. method:: AbstractEventLoop.call_at(when, callback, *args)
+   .. versionchanged:: 3.7
+      The *context* keyword-only parameter was added. See :pep:`567`
+      for more details.
+
+.. method:: AbstractEventLoop.call_at(when, callback, *args, context=None)
 
    Arrange for the *callback* to be called at the given absolute timestamp
    *when* (an int or float), using the same time reference as
@@ -199,6 +219,10 @@
    :ref:`Use functools.partial to pass keywords to the callback
    <asyncio-pass-keywords>`.
 
+   .. versionchanged:: 3.7
+      The *context* keyword-only parameter was added. See :pep:`567`
+      for more details.
+
 .. method:: AbstractEventLoop.time()
 
    Return the current time, as a :class:`float` value, according to the