Clarify that pusher URLs have a fixed path (#1974)

Co-authored-by: Andrew Morgan <[email protected]>

changelogs/push_gateway/newsfragments/1974.clarification

@@ -0,0 +1 @@
+The path of HTTP pusher URLs is fixed to `/_matrix/push/v1/notify`.

data/api/push-gateway/push_notifier.yaml

@@ -35,9 +35,9 @@ paths:
         updating counts of unread notifications should be idempotent and
         therefore do not require duplicate suppression.
 
-        Notifications are sent to the URL configured when the pusher is created.
-        This means that the HTTP path may be different depending on the push
-        gateway.
+        Clients interested in receiving notifications via this endpoint MUST
+        configure its full URI when creating the associated pusher via
+        [`/_matrix/client/v3/pushers/set`](/client-server-api/#post_matrixclientv3pushersset).
       operationId: notify
       requestBody:
         content: