JavaSDK Activity Cancellation section needs a snippet and more details

[Spikhalskiy]

https://docs.temporal.io/docs/java/activities/#activity-cancellation

1. Needs an improvement with a code snipped of activity getting information about cancellation from a heartbeat request using ActivityCompletionException
2. Needs an explanation when and why Activity should rethrow this exception and how it's connected with ActivityCancellationType set on ActivityOptions: https://github.com/temporalio/sdk-java/blob/4d90cf0dc644646998135aa5e6b2a59fe13303de/temporal-sdk/src/main/java/io/temporal/activity/ActivityCancellationType.java#L29

[flossypurse]

-> How to cancel an Activity Execution

[axfelix]

We don't really talk about cancelling in current docs as much as we talk about heartbeats etc -- is this still planned, @flossypurse ?