Add documentation

Author: sebromero

examples/ModemTerminal/ModemTerminal.ino

@@ -17,6 +17,7 @@ void setup(){
     Serial.println("Connecting...");
     cellular.connect(SECRET_GPRS_APN, SECRET_GPRS_LOGIN, SECRET_GPRS_PASSWORD);
     Serial.println("Connected!");
+    Serial.println("You can now send AT commands to the modem.");
 }
 
 void loop() {

src/ArduinoCellular.h

@@ -179,7 +179,7 @@ class ArduinoCellular {
         /**
          * @brief Sends an AT command to the modem and waits for a response, then returns the response.
          * @param command The AT command to send.
-         * @param timeout The timeout (In milliseconds) to wait for the response.
+         * @param timeout The timeout (In milliseconds) to wait for the response. Default is 1000ms.
          * @return The response from the modem.
          */
         String sendATCommand(const char * command, unsigned long timeout = 1000);