Add AsyncContextManager to typing module documentation. (GH-6822)

(cherry picked from commit b7b493e)

Co-authored-by: Travis DePrato <[email protected]>

Author: miss-islington

Doc/library/typing.rst

@@ -666,6 +666,12 @@ The module defines the following classes, functions and decorators:
 
    .. versionadded:: 3.6
 
+.. class:: AsyncContextManager(Generic[T_co])
+
+   A generic version of :class:`contextlib.AbstractAsyncContextManager`.
+
+   .. versionadded:: 3.6
+
 .. class:: Dict(dict, MutableMapping[KT, VT])
 
    A generic version of :class:`dict`.

Misc/NEWS.d/next/Documentation/2018-05-14-15-15-41.bpo-33421.3GU_QO.rst

@@ -0,0 +1 @@
+Add missing documentation for ``typing.AsyncContextManager``.