Add notes about WLAN and WDT constructor that can be used to retrieve the existing instance(s).

Hardware-API.md

@@ -371,6 +371,9 @@ Methods:
 - `wdt.feed()` feed the watchdog.
 - `wdt.deinit()` disable the WDT (again, might not be possible on some platforms, raise `OSError` in that case).
 
+**Note:**
+_danicampora_: Just as WLAN below, I think it should be possible to retrieve the existing WDT instance when calling the constructor with no params besides the id.
+
 ## The WLAN class
 
 The WLAN class belongs to the network module.
@@ -379,6 +382,10 @@ Constructor:
 
 `wlan = WLAN(id, mode=WLAN.STA, *, ssid='wlan', auth=None, channel=1, iface=None)`
 
+**Note:**
+Since WLAN might be a system feature of the platform (this is the case of the WiPy and the ESP8266), 
+calling the constructor without params (besides the id) will return the existing WLAN object.
+
 Methods:
 
 - `wlan.init()` re-init.