docs(auth): added onAuthStateChanged to the signInWithPhoneNumber Example

related to #530

Author: Bilal-Abdeen

docs/auth/phone-auth.md

@@ -62,8 +62,24 @@ function PhoneSignIn() {
   // If null, no SMS has been sent
   const [confirm, setConfirm] = useState(null);
 
+  // verification code (OTP - One-Time-Passcode)
   const [code, setCode] = useState('');
 
+  // Handle login
+  function onAuthStateChanged(user) {
+    if (user) {
+      // Some Android devices can automatically process the verification code (OTP) message, and the user would NOT need to enter the code.
+      // Actually, if he/she tries to enter it, he/she will get an error message because the code was already used in the background.
+      // In this function, make sure you hide the component(s) for entering the code and/or navigate away from this screen.
+      // It is also recommended to display a message to the user informing him/her that he/she has successfully logged in.
+    }
+  }
+
+  useEffect(() => {
+    const subscriber = auth().onAuthStateChanged(onAuthStateChanged);
+    return subscriber; // unsubscribe on unmount
+  }, []);
+
   // Handle the button press
   async function signInWithPhoneNumber(phoneNumber) {
     const confirmation = await auth().signInWithPhoneNumber(phoneNumber);