graphql-ws 5.6.1 fixes its multi subprotocols problem. Add a mention in the docs

Author: leszekhanusz

docs/transports/websockets.rst

@@ -12,7 +12,7 @@ It will propose both subprotocols to the backend and detect the supported protoc
 from the response http headers returned by the backend.
 
 .. note::
-    For some backends (graphql-ws without backwards compatibility), it may be necessary to specify
+    For some backends (graphql-ws before `version 5.6.1`_ without backwards compatibility), it may be necessary to specify
     only one subprotocol to the backend. It can be done by using
     :code:`subprotocols=[WebsocketsTransport.GRAPHQLWS_SUBPROTOCOL]`
     or :code:`subprotocols=[WebsocketsTransport.APOLLO_SUBPROTOCOL]` in the transport arguments.
@@ -125,5 +125,6 @@ Here is an example with a ping sent every 60 seconds, expecting a pong within 10
         pong_timeout=10,
     )
 
+.. _version 5.6.1: https://github.com/enisdenjo/graphql-ws/releases/tag/v5.6.1
 .. _Apollo websockets transport protocol:  https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md
 .. _GraphQL-ws websockets transport protocol: https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md