asyncio.docs: Document Transport.is_closing (merge 3.4)

diff --cc Doc/library/asyncio-protocol.rst
index 9264343db6070af6f80bf1aa3be7ef539253b163,9350ffedeb7edda3e519c4b7601aa72b71e8f7d9..295f1edc12c14591ad521083fac1645a7798b7c9
--- 1/Doc/library/asyncio-protocol.rst
--- 2/Doc/library/asyncio-protocol.rst
+++ b/Doc/library/asyncio-protocol.rst
@@@ -41,6 -41,11 +41,11 @@@ BaseTranspor
        protocol's :meth:`connection_lost` method will be called with
        :const:`None` as its argument.
  
 -      .. versionadded:: 3.4.4
+    .. method:: is_closing(self)
+ 
+       Return ``True`` if the transport is closing or is closed.
+ 
++      .. versionadded:: 3.5.1
  
     .. method:: get_extra_info(name, default=None)