Added a small note about Webpack server and Symfony server

Author: javiereguiluz

frontend/encore/dev-server.rst

@@ -24,6 +24,12 @@ by the normal `webpack-dev-server`_. For example:
 
 This will start a server at ``https://localhost:9000``.
 
+.. note::
+
+    This Webpack server is independent from
+    :doc:`Symfony's development web server </setup/built_in_web_server>` and
+    you need to run both separately.
+
 Using dev-server inside a VM
 ----------------------------