Update comments in examples (#457)

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

examples/rfid_read_personal_data/rfid_read_personal_data.ino

@@ -49,7 +49,7 @@ void loop() {
 
   //-------------------------------------------
 
-  // Look for new cards
+  // Reset the loop if no new card present on the sensor/reader. This saves the entire process when idle.
   if ( ! mfrc522.PICC_IsNewCardPresent()) {
     return;
   }