[Breaking] Add disconnect reason to client callback. (#398) (#81)

Adds the reason code as a parameter to the client onDisconnect callback.

* Update examples/docs.

docs/Migration_guide.md

@@ -16,6 +16,7 @@ For more information on the improvements and additions please refer to the [clas
 * [Client](#client-api)
     * [Remote Services](#remote-services)
     * [Remote characteristics](#remote-characteristics)
+    * [Client Callbacks](#client-callbacks)
     * [Security](#client-security)
 * [Scanning](#scan-api)
 * [General Security](#security-api)
@@ -316,6 +317,13 @@ the currently known database returned (false : default).
 Also now returns a pointer to `std::vector` instead of `std::map`.
 <br/>
 
+<a name="client-callbacks"></a>
+### Client callbacks
+
+> `BLEClientCallbacks::onDisconnect` (`NimBLEClientCallbacks::onDisconnect`)
+
+This now takes a second parameter `int reason` which provides the reason code for disconnection.
+
 <a name="client-security"></a>
 ### Client Security
 The client will automatically initiate security when the peripheral responds that it's required.