Merge pull request #822 from arduino/attach-interrupt-fix

Updated mega row for interrupts

Author: kengdahl

Language/Functions/External Interrupts/attachInterrupt.adoc

@@ -22,7 +22,7 @@ The first parameter to `attachInterrupt()` is an interrupt number. Normally you
 |Board                             |Digital Pins Usable For Interrupts
 |Uno, Nano, Mini, other 328-based  |2, 3
 |Uno WiFi Rev.2, Nano Every        |all digital pins
-|Mega, Mega2560, MegaADK           |2, 3, 18, 19, 20, 21
+|Mega, Mega2560, MegaADK           |2, 3, 18, 19, 20, 21 (*pins 20 & 21* are not available to use for interrupts while they are used for I2C communication)
 |Micro, Leonardo, other 32u4-based |0, 1, 2, 3, 7
 |Zero                              |all digital pins, except 4
 |MKR Family boards                 |0, 1, 4, 5, 6, 7, 8, 9, A1, A2