[tutorial 011] Describe the variable

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/stove/rougail-user-data-questionary)).

firefox/00-proxy.yml

@@ -2,5 +2,5 @@
 ---
 version: 1.1
 
-proxy_mode:
+proxy_mode:  # Configure Proxy Access to the Internet
 ...