chore: Clarify secret usage with comments

examples/ConnectionHandlerDemo/arduino_secrets.h

@@ -1,14 +1,18 @@
+// Required for WiFiConnectionHandler
 const char SECRET_SSID[] = "NETWORK NAME";
 const char SECRET_PASS[] = "NETWORK PASSWORD";
 
+// Required for GSMConnectionHandler
 const char SECRET_APN[] 		= "MOBILE PROVIDER APN ADDRESS";
-const char SECRET_PIN[] 		= "0000";
+const char SECRET_PIN[] 		= "0000";  // Required for NBConnectionHandler
 const char SECRET_GSM_USER[] 	= "GSM USERNAME";
 const char SECRET_GSM_PASS[]	= "GSM PASSWORD";
 
+// Required for LoRaConnectionHandler
 const char SECRET_APP_EUI[]  = "APP_EUI";
 const char SECRET_APP_KEY[]  = "APP_KEY";
 
+// Required for EthernetConnectionHandler (without DHCP mode)
 const char SECRET_IP[]       = "IP ADDRESS";
 const char SECRET_DNS[]      = "DNS ADDRESS";
 const char SECRET_GATEWAY[]  = "GATEWAY ADDRESS";