Add extra documentation on asBroadcastStream.
Add extra documentation on asBroadcastStream.

Fixes #30395
BUG= https://github.com/dart-lang/sdk/issues/30395

Change-Id: I887b4b5882b9f4801f29690c2d039941eb99efd1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201567
Auto-Submit: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Reviewed-by: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk