feat: add missing Subscription API descriptions (#4044)

Fixes #4008

Co-authored-by: Anton Platonov <anton@vaadin.com>

Author: taefi

articles/hilla/lit/guides/reactive-endpoints.adoc

@@ -176,4 +176,10 @@ Be informed when the Flux has no more messages to send.
 [methodname]`context()`::
 Bind to the context, which allows the application automatically to close it when the view is detached.
 
+[methodname]`onSubscriptionLost()`::
+Called when the connection is restored, but there's no longer a valid subscription. If the callback returns `ActionOnLostSubscription.RESUBSCRIBE`, the subscription will be re-established by connecting to the same server method again. If the callback returns `ActionOnLostSubscription.REMOVE`, the subscription will be forgotten. This is also the default behavior if the callback is not set or if it returns `undefined`.
+
+[methodname]`onConnectionStateChange()`::
+Called when the subscription state changes. The callback receives an event of type `FluxSubscriptionStateChangeEvent` that contains the new state. The possible states are defined as enumerated values of `CONNECTING`, `CONNECTED`, and `CLOSED`.
+
 // end::content[]