Add destroy to docs (#411)

Just found this missing function.

docs/docs_client.rst

@@ -185,6 +185,14 @@ logout(`callback`)
 
 Logs the client out and closes the WebSocket connections.
 
+- **callback** - `function` that takes the following parameter:
+    - **error** - An error if any occurred
+
+destroy(`callback`)
+~~~~~~~~~~~~~~~~~~~
+
+Similar to logout but should be used if you're not going to create the Client again later in your program.
+
 - **callback** - `function` that takes the following parameter:
     - **error** - An error if any occurred