Update comments in examples (#457)

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

examples/rfid_write_personal_data/rfid_write_personal_data.ino

@@ -40,7 +40,7 @@ void loop() {
   MFRC522::MIFARE_Key key;
   for (byte i = 0; i < 6; i++) key.keyByte[i] = 0xFF;
 
-  // 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;
   }
@@ -153,4 +153,4 @@ void loop() {
   mfrc522.PICC_HaltA(); // Halt PICC
   mfrc522.PCD_StopCrypto1();  // Stop encryption on PCD
 
-}
+}