Watchdog: add docs update about intention one vs multiple Watchdogs

VirtualWatchdog - multiple timers in the system - most used watchdog
Watchdog - simple driver for one system timer in the app - bootloader or similar

Author: 0xc0170

drivers/VirtualWatchdog.h

@@ -36,7 +36,10 @@ namespace mbed {
 
 /** \addtogroup drivers */
 
-/** A system timer that will reset the system in the case of system failures or
+/** VirtualWatchdog should be used for applications where you use multiple services requiring watchdog functionality.
+ *   Use `Watchdog` driver for simple uses cases like bootloader.
+ *
+ *  A system timer that will reset the system in the case of system failures or
  *  malfunctions.
  *
  * Example:

drivers/Watchdog.h

@@ -31,7 +31,7 @@ namespace mbed {
 
 /** \addtogroup drivers */
 /** Hardware system timer that will reset the system in the case of system failures or
- *  malfunctions. There is only one instance in the system.
+ *  malfunctions. There is only one instance in the system (for multiple system timers, see VirtualWatchdog driver).
  *
  * Example:
  * @code