Cellular: Added explanatory comment for AT cellular stack's socket_open routine

Author: Mirela Chirica

features/cellular/framework/AT/AT_CellularStack.h

@@ -52,6 +52,12 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
      */
     virtual nsapi_error_t socket_stack_init();
 
+    /**
+      * Note: Socket_open does not actually open socket on all drivers, but that's deferred until calling `sendto`.
+      * The reason is that IP stack implementations are very much modem specific and it's quite common that when a
+      * socket is created (via AT commands) it must also be given remote IP address, and that is usually known
+      * only when calling `sendto`.
+      */
     virtual nsapi_error_t socket_open(nsapi_socket_t *handle, nsapi_protocol_t proto);
 
     virtual nsapi_error_t socket_close(nsapi_socket_t handle);