Update documentation

lib/mailbox.dart

@@ -68,7 +68,8 @@ class Mailbox {
 
   /// Place a message into the mailbox if has space for it.
   ///
-  /// If mailbox already contains a message then [put] will throw.
+  /// If mailbox already contains a message or mailbox is closed then [put] will
+  /// throw [StateError].
   void put(Uint8List message) {
     final buffer = message.isEmpty ? nullptr : _toBuffer(message);
     _mutex.runLocked(() {
@@ -102,7 +103,7 @@ class Mailbox {
   /// Take a message from the mailbox.
   ///
   /// If mailbox is empty this will synchronously block until message
-  /// is available.
+  /// is available. If mailbox is closed [take] will throw [StateError].
   Uint8List take() => _mutex.runLocked(() {
         while (_mailbox.ref.state == _stateEmpty) {
           _condVar.wait(_mutex);