Update /pushers/set info to make deletion clearer

I had trouble figuring out how to remove a pusher with this endpoint today, and had to look synapse's implementation to figure it out. Later @richvdh  pointed out that this bit of info was indeed already there, in the `kind` property: "null deletes the pusher.", but I feel it would still be valuable to add this info to the top and specify what additional params required for the operation (at least according to synapse). 
Maybe the text I added needs some changes to make it more align with the pattern from the rest of the specs, let me know.

data/api/client-server/pusher.yaml

@@ -139,6 +139,7 @@ paths:
         This endpoint allows the creation, modification and deletion of [pushers](/client-server-api/#push-notifications)
         for this user ID. The behaviour of this endpoint varies depending on the
         values in the JSON body.
+        For deletion, send a request with`kind:null`, including `app_id` and `pushkey` of the pusher you want to remove. 
       operationId: postPusher
       security:
         - accessToken: []