Minor doc update

Socket ID is usually spitted out by the modem, however there are cases
when the modem is actully taken in as an input argument, e.g., in the
case of QUECTEL M26 modem. This minor knit clarifies that the
CellularSocket::id can be an input argument.

features/cellular/framework/AT/AT_CellularStack.h

@@ -105,7 +105,8 @@ protected:
             pending_bytes(0)
         {
         }
-        // Socket id from cellular device
+        // Socket identifier, generally it will be the socket ID assigned by the
+        // modem. In a few special cases, modems may take that as an input argument.
         int id;
         // Being connected means remote ip address and port are set
         bool connected;