Expand StreamTransform.fromHandlers docs

Closes #27740

Mention the convenience constructors `fromhandlers` and `fromBind` in
the class level doc.

Describe the default behavior of each handlers.

R=lrn@google.com

Change-Id: Ica55d5a2f531e11106085122aa32173cc436991a
CoreLibraryReviewExempt: Doc changes only.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310765
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

https://dart.googlesource.com/sdk/+/d3f79404ac7ca8d5a2156fa494cd07c3f6d8d92e
1 file changed
tree: f46fd275a5386b8077b616f5bf59c41872ddaefc
  1. commits.json