add missing information about how to get the cancelation request in an activity in TS

[antmendoza]

Temporal Affiliation

• I am a Temporal employee

Page URL

https://docs.temporal.io/develop/typescript/failure-detection#activity-timeouts

Content Issue

https://github.com/temporalio/documentation/blob/main/docs/develop/typescript/failure-detection.mdx

Activity Cancellations are delivered to Activities from the Temporal Service when they Heartbeat. Activities that don't Heartbeat can't get notified of Cancellation requests.

We should mention that (in typescript) to get the cancellation request in an activity, users have to check the cancellation promise/signal of the context (Context.current().cancelled) or use activity.sleep

Or link this source https://typescript.temporal.io/api/namespaces/activity#cancellation