Add documentation for PeriodMultiplier (#632)

wpilibj/src/main/java/edu/wpi/first/wpilibj/PWM.java

@@ -20,7 +20,7 @@ import edu.wpi.first.wpilibj.tables.ITableListener;
  *
  * <p>The values supplied as arguments for PWM outputs range from -1.0 to 1.0. They are mapped to
  * the hardware dependent values, in this case 0-2000 for the FPGA. Changes are immediately sent to
- * the FPGA, and the update occurs at the next FPGA cycle. There is no delay.
+ * the FPGA, and the update occurs at the next FPGA cycle (5.005ms). There is no delay.
  *
  * <p>As of revision 0.1.10 of the FPGA, the FPGA interprets the 0-2000 values as follows: - 2000 =
  * maximum pulse width - 1999 to 1001 = linear scaling from "full forward" to "center" - 1000 =
@@ -33,15 +33,15 @@ public class PWM extends SensorBase implements LiveWindowSendable {
    */
   public enum PeriodMultiplier {
     /**
-     * Period Multiplier: don't skip pulses.
+     * Period Multiplier: don't skip pulses. PWM pulses occur every 5.005 ms
      */
     k1X,
     /**
-     * Period Multiplier: skip every other pulse.
+     * Period Multiplier: skip every other pulse. PWM pulses occur every 10.010 ms
      */
     k2X,
     /**
-     * Period Multiplier: skip three out of four pulses.
+     * Period Multiplier: skip three out of four pulses. PWM pulses occur every 20.020 ms
      */
     k4X
   }