docs/esp8266: Update PWM doc regarding clipping of min/max values.

docs/esp8266/tutorial/pwm.rst

@@ -28,8 +28,8 @@ You can set the frequency and duty cycle using::
     >>> pwm12.duty(512)
 
 Note that the duty cycle is between 0 (all off) and 1023 (all on), with 512
-being a 50% duty.  If you print the PWM object then it will tell you its current
-configuration::
+being a 50% duty. Values beyond this min/max will be clipped. If you
+print the PWM object then it will tell you its current configuration::
 
     >>> pwm12
     PWM(12, freq=500, duty=512)