[vm] Document the new implementation of async/async*/sync* based on suspend/resume stubs

TEST=none (documentation only change)

Issue: #48378
Change-Id: I61e734f85c40e3a301c4185e8917e78c37171590
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251680
Reviewed-by: Ryan Macnak <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov