Revert session.close() in docs back to coroutine

Author: asvetlov

docs/client_reference.rst

@@ -474,14 +474,12 @@ The client session supports the context manager protocol for self closing.
 
          URLs may be either :class:`str` or :class:`~yarl.URL`
 
-   .. method:: close()
+   .. comethod:: close()
 
       Close underlying connector.
 
       Release all acquired resources.
 
-      .. versionchanged:: 2.0
-
    .. method:: detach()
 
       Detach connector from session without closing the former.

docs/migration.rst

@@ -140,7 +140,6 @@ Various
    does not match, it raises :exc:`aiohttp.ClientResponseError` exception.
    To disable content type check you can pass ``None`` as `content_type` parameter.
 
-7. `ClientSession.close()` is a regular function returning None, not a coroutine.