Clarify concurrency in Trans methods

connection.go

@@ -28,23 +28,31 @@ type Conn interface {
 	SetSizeLimit(limit int64)
 }
 
-// Trans is a HOPP transaction.
+// Trans is a HOPP transaction. Methods of this interface are not safe for
+// concurrent use with the exception of the Close and ID methods. The
+// recommended use case is one goroutine per transaction.
 type Trans interface {
 	// Close closes the transaction. Any blocked operations will be
-	// unblocked and return errors.
+	// unblocked and return errors. This method is safe for concurrent use.
 	Close() error
 
 	// ID returns the transaction ID. This must not change, and it must be
-	// unique within the connection.	
+	// unique within the connection. This method is safe for concurrent use.
 	ID() int64
 
 	// TODO: add methods for setting send and receive deadlines
 		
-	// Send sends a message.
+	// Send sends a message. This method is not safe for concurrent use.
 	Send(method uint16, data []byte) error
-	// Receive receives a message.
+	// SendWriter sends data written to an [io.Writer]. Any writer
+	// previously opened through this function will be discarded. This
+	// method is not safe for concurrent use, and neither is its result.
+	SendWriter(method uint16) (io.Writer, error)
+	// Receive receives a message. This method is not safe for concurrent
+	// use.
 	Receive() (method uint16, data []byte, err error)
 	// ReceiveReader receives a message as an [io.Reader]. Any reader
-	// previously opened through this function will be discarded.
+	// previously opened through this function will be discarded. This
+	// method is not safe for concurrent use, and neither is its result.
 	ReceiveReader() (method uint16, data io.Reader, err error)
 }