Update AV1820: Only use async for I/O-bound or long-running activities (#376)

Author: dennisdoomen

_rules/1820.md

@@ -1,7 +1,9 @@
 ---
 rule_id: 1820
 rule_category: performance
-title: Only use `async` for low-intensive long-running activities
+title: Only use `async`/`await` for I/O-bound or long-running activities
 severity: 1
 ---
-The usage of `async` won't automagically run something on a worker thread like `Task.Run` does. It just adds the necessary logic to allow releasing the current thread, and marshal the result back on that same thread if a long-running asynchronous operation has completed. In other words, use `async` only for I/O bound operations.
+The use of `async`/`await` won't automagically run something on a worker thread as `Task.Run` does. It just suspends execution at the await point and resumes execution after the task has completed. In other words, use `async`/`await` only for I/O-bound operations.
+
+**Exception:** Tasks returned from `Task.Run` (which starts a background operation in parallel) can eventually be awaited to obtain their results, or passed to a method like `Task.WhenAll` that is awaited.