Call out missed events in startWith Doc (dart-lang/stream_transform#151)

The listen will not happen until the first event is delivered.

pkgs/stream_transform/lib/src/concatenate.dart

diff --git a/pkgs/stream_transform/lib/src/concatenate.dart b/pkgs/stream_transform/lib/src/concatenate.dart
index 3277dcd..0330dd7 100644
--- a/pkgs/stream_transform/lib/src/concatenate.dart
+++ b/pkgs/stream_transform/lib/src/concatenate.dart
@@ -82,6 +82,9 @@
   /// Emits [initial] before any values or errors from the this stream.
   ///
   /// If this stream is a broadcast stream the result will be as well.
+  /// If this stream is a broadcast stream, the returned stream will only
+  /// contain events of this stream that are emitted after the [initial] value
+  /// has been emitted on the returned stream.
   Stream<T> startWith(T initial) =>
       startWithStream(Future.value(initial).asStream());