Update comments and Readme.

Author: mobizt

README.md

@@ -525,6 +525,8 @@ param **`networkConnectionCB`** The function that handles the network connection
 
 param **`networkStatusCB`** The function that handle the network connection status acknowledgement.
 
+Due to the client pointer is assigned, to avoid dangling pointer, client should be existed as long as it was used for transportation.
+
 ```cpp
 void setExternalClient(Client *client, GS_NetworkConnectionRequestCallback networkConnectionCB,
                            GS_NetworkStatusRequestCallback networkStatusCB);
@@ -545,6 +547,8 @@ param **`user`** The GPRS user.
 
 param **`password`** The GPRS password.
 
+Due to the client and modem pointers are assigned, to avoid dangling pointer, client should be existed as long as it was used for transportation.
+
 ```cpp
 void setGSMClient(Client *client, void *modem, const char *pin, const char *apn, const char *user, const char *password);
 ```

examples/Ethernet/Pico/Pico.ino

@@ -151,6 +151,8 @@ void setupGsheet()
     GSheet.setTokenCallback(tokenStatusCallback);
 
     /* Assign the pointer to global defined external SSL Client object and required callback functions */
+    // Due to the client (eth) pointer is assigned, to avoid dangling pointer, 
+    // client should be existed as long as it was used for transportation.
     GSheet.setExternalClient(&eth, networkConnection, networkStatusRequestCallback);
 
     // Set the seconds to refresh the auth token before expire (60 to 3540, default is 300 seconds)

examples/GSM/GSM.ino

@@ -145,7 +145,9 @@ void setup()
     DBG("Modem Info:", modemInfo);
 
     GSheet.printf("ESP Google Sheet Client v%s\n\n", ESP_GOOGLE_SHEET_CLIENT_VERSION);
-
+    
+    // Due to the gsm_client and modem pointers are assigned, to avoid dangling pointer, 
+    // gsm_client and modem should be existed as long as it was used for transportation.
     GSheet.setGSMClient(&gsm_client, &modem, GSM_PIN, apn, gprsUser, gprsPass);
 }
 

src/ESP_Google_Sheet_Client.h

@@ -1468,6 +1468,9 @@ class ESP_Google_Sheet_Client
      * @param client The pointer to Arduino Client derived class of SSL Client.
      * @param networkConnectionCB The function that handles the network connection.
      * @param networkStatusCB The function that handle the network connection status acknowledgement.
+     * 
+     * Due to the client pointer is assigned, to avoid dangling pointer, 
+     * client should be existed as long as it was used for transportation.
      */
     void setExternalClient(Client *client, ESP_GOOGLE_SHEET_CLIENT_NetworkConnectionRequestCallback networkConnectionCB,
                            ESP_GOOGLE_SHEET_CLIENT_NetworkStatusRequestCallback networkStatusCB)
@@ -1483,6 +1486,9 @@ class ESP_Google_Sheet_Client
      * @param apn The GPRS APN (Access Point Name).
      * @param user The GPRS user.
      * @param password The GPRS password.
+     * 
+     * Due to the client and modem pointers are assigned, to avoid dangling pointer, 
+     * client and modem should be existed as long as it was used for transportation.
      */
     void setGSMClient(Client *client, void *modem = nullptr, const char *pin = nullptr, const char *apn = nullptr, const char *user = nullptr, const char *password = nullptr)
     {