apache frontend: describe a bit more disabled-cookie-list parameter

Quoting Romain:
> As you can see at https://lab.nexedi.com/nexedi/erp5/blob/master/bt5/erp5_web/ToolTemplateItem/caching_policy_manager.xml
> , caching policy manager often set Cookie in the Vary  header.
> For the CDN, it means that each query on the same URL but with a
> different cookie will never be a HIT, and so, query will always access
> ERP5 backend.

software/apache-frontend/instance-slave-apache-input-schema.json

@@ -125,7 +125,7 @@
 
     "disabled-cookie-list": {
       "title": "Disabled Cookies",
-      "description": "List of Cookies separated by space that will not be sent to the backend",
+      "description": "List of Cookies separated by space that will not be sent to cache and backend servers. This is especially useful to discard analytics tracking cookies when using Vary: Cookie cache headers",
       "type": "string",
       "default": ""
     },