[docs] Add a note about reconnecting after a server-side disconnection

docs/API.md

@@ -603,7 +603,11 @@ Fired upon a disconnection.
 
 ```js
 socket.on('disconnect', (reason) => {
-  // ...
+  if (reason === 'io server disconnect') {
+    // the disconnection was initiated by the server, you need to reconnect manually
+    socket.connect();
+  }
+  // else the socket will automatically try to reconnect
 });
 ```