Update millis.adoc

Use of millis() may become unreliable if timer0 is reprogrammed, so a small warning might be useful that timer0 is used by millis().

Author: Arnold-n

Language/Functions/Time/millis.adoc

@@ -64,7 +64,7 @@ void loop() {
 
 [float]
 === Notes and Warnings
-Please note that the return value for millis() is of type `unsigned long`, logic errors may occur if a programmer tries to do arithmetic with smaller data types such as `int`. Even signed `long` may encounter errors as its maximum value is half that of its unsigned counterpart.
+Please note that the return value for millis() is of type `unsigned long`, logic errors may occur if a programmer tries to do arithmetic with smaller data types such as `int`. Even signed `long` may encounter errors as its maximum value is half that of its unsigned counterpart. millis() uses timer0's overflow interrupt.
 
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 // HOW TO USE SECTION ENDS