Add documentation for best practices for StreamBuilder like FutureBuilder (#143295)

Fixes flutter/flutter#142189

Author: ValentinVignal

packages/flutter/lib/src/widgets/async.dart

@@ -321,6 +321,20 @@ typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnaps
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=MkKEWHfy99Y}
 ///
+/// ## Managing the stream
+///
+/// The [stream] must have been obtained earlier, e.g. during [State.initState],
+/// [State.didUpdateWidget], or [State.didChangeDependencies]. It must not be
+/// created during the [State.build] or [StatelessWidget.build] method call when
+/// constructing the [StreamBuilder]. If the [stream] is created at the same
+/// time as the [StreamBuilder], then every time the [StreamBuilder]'s parent is
+/// rebuilt, the asynchronous task will be restarted.
+///
+/// A general guideline is to assume that every `build` method could get called
+/// every frame, and to treat omitted calls as an optimization.
+///
+/// ## Timing
+///
 /// Widget rebuilding is scheduled by each interaction, using [State.setState],
 /// but is otherwise decoupled from the timing of the stream. The [builder]
 /// is called at the discretion of the Flutter pipeline, and will thus receive a