VirtualWatchdog: add config option to ctor to clarify the hw timeout

drivers/VirtualWatchdog.cpp

@@ -36,6 +36,7 @@ VirtualWatchdog::VirtualWatchdog(uint32_t timeout, const char *const str): _name
         if (watchdog.is_running() == true) {
             MBED_MAKE_ERROR(MBED_MODULE_DRIVER_WATCHDOG, MBED_ERROR_INITIALIZATION_FAILED);
         }
+        // we use default hw timeout provided by config
         watchdog.start(&VirtualWatchdog::process, Watchdog::watchdog_timeout);
         _is_hw_watchdog_running = true;
     }

drivers/VirtualWatchdog.h

@@ -54,6 +54,9 @@ class VirtualWatchdog {
 public:
 
     /** Constructor configured with timeout and name for this software watchdog instance.
+     *
+     * We start hardware watchdog on  the first run. This set-up uses default watchdog timeout set via config value
+     * MBED_CONF_TARGET_WATCHDOG_TIMEOUT.
      *
      */
     VirtualWatchdog(uint32_t timeout = 1, const char *const str = NULL);