ADd some docs for the scan cycle timer

src/kaleidoscope/driver/keyscanner/ATmega.h

@@ -75,10 +75,15 @@ class ATmega: public kaleidoscope::driver::keyscanner::Base<_KeyScannerProps> {
       OUTPUT_HIGH(_KeyScannerProps::matrix_row_pins[i]);
     }
 
-    /* Set up Timer1 for 1700usec */
     setScanCycleTime(_KeyScannerProps::keyscan_interval);
   }
 
+
+  /* setScanCycleTime takes a value of between 0 and 8192. This corresponds (roughly) to the number of microseconds to wait between scanning the key matrix. Our debouncing algorithm does four checks before deciding that a result is valid. Most normal mechanical switches specify a 5ms debounce period. On an ATMega32U4, 1700 gets you about 5ms of debouncing.
+
+Because keycanning is triggered by an interrupt but not run in that interrupt, the actual amount of time between scans is prone to a little bit of jitter.
+
+ */
   void setScanCycleTime(uint16_t c) {
     TCCR1B = _BV(WGM13);
     TCCR1A = 0;