[tutorial 004] The variable description

We can add a description to this first variable.

This information is useful for:

- documentation
- error message
- help user to known which value to set to this variable (for example with [the user data plugin Questionary](https://forge.cloud.silique.fr/gnunux/rougail-user-data-questionary)).

firefox/00-proxy.yml

@@ -1,2 +1,3 @@
 ---
 proxy_mode:
+  description: Configure Proxy Access to the Internet