Warn about increasing HardwareSerial buffers to > 256

Properly supporting bigger buffers need better atomicity code, which
isn't implemented yet. Until it is, best to add a warning. See #2405.

hardware/arduino/avr/cores/arduino/HardwareSerial.h

@@ -34,6 +34,11 @@
 // location from which to read.
 // NOTE: a "power of 2" buffer size is reccomended to dramatically
 //       optimize all the modulo operations for ring buffers.
+// WARNING: When buffer sizes are increased to > 256, the buffer index
+// variables are automatically increased in size, but the extra
+// atomicity guards needed for that are not implemented. This will
+// often work, but occasionally a race condition can occur that makes
+// Serial behave erratically. See https://github.com/arduino/Arduino/issues/2405
 #if !defined(SERIAL_TX_BUFFER_SIZE)
 #if (RAMEND < 1000)
 #define SERIAL_TX_BUFFER_SIZE 16