Be more explicit about JSON error handling.

lib/src/json_document_transformer.dart

Index: lib/src/json_document_transformer.dart
diff --git a/lib/src/json_document_transformer.dart b/lib/src/json_document_transformer.dart
index 8d8dcce231b68cbdbe14b8bc308491a700ced464..19b2e08f75e0f4818125ed8b67a0386501a409e9 100644
--- a/lib/src/json_document_transformer.dart
+++ b/lib/src/json_document_transformer.dart
@@ -17,6 +17,10 @@ final jsonDocument = new JsonDocumentTransformer();
 ///
 /// This decodes JSON that's emitted by the transformed channel's stream, and
 /// encodes objects so that JSON is passed to the transformed channel's sink.
+///
+/// If the transformed channel emits invalid JSON, this emits a
+/// [FormatException]. If an unencodable object is added to the sink, it
+/// synchronously throws a [JsonUnsupportedObjectError].
 class JsonDocumentTransformer
     implements StreamChannelTransformer<String, Object> {
   /// The underlying codec that implements the encoding and decoding logic.