Update comments in examples (#457)

It's seems not that important, but this helps the new user understand the code.

examples/ChangeUID/ChangeUID.ino

@@ -61,7 +61,7 @@ void setup() {
 // But of course this is a more proper approach
 void loop() {
   
-  // Look for new cards, and select one if present
+  // Reset the loop if no new card present on the sensor/reader. This saves the entire process when idle. And if present, select one.
   if ( ! mfrc522.PICC_IsNewCardPresent() || ! mfrc522.PICC_ReadCardSerial() ) {
     delay(50);
     return;