WIP: documentation change

src/machine/uart.go

@@ -26,11 +26,12 @@ const (
 // To implement the UART interface for a board, you must declare a concrete type as follows:
 //
 // 		type UART struct {
-// 			Buffer *RingBuffer
+// 			Buffer     *RingBuffer
+// 			rxCallback func(byte) bool
 // 		}
 //
 // You can also add additional members to this struct depending on your implementation,
-// but the *RingBuffer is required.
+// Buffer and rxCallback are required.
 // When you are declaring your UARTs for your board, make sure that you also declare the
 // RingBuffer using the NewRingBuffer() function when you declare your UART:
 //