Add an explanatory comment

Signed-off-by: Jesse Vincent <jesse@keyboard.io>

plugins/Kaleidoscope-FocusSerial/src/kaleidoscope/plugin/FocusSerial.cpp

@@ -33,6 +33,7 @@ void FocusSerial::drain(void) {
 }
 
 EventHandlerResult FocusSerial::afterEachCycle() {
+  // If the serial buffer is empty, we don't have any work to do
   if (Runtime.serialPort().available() == 0)
     return EventHandlerResult::OK;