documentation improvement: effect of APP_DOMAIN on session cookie configuration

[gwimmel]

In CONFIGURATION.md, it says that APP_DOMAIN is "Used for creating URLs in emails.". Therefore, one cold conclude that it is irrelevant when the email feature is not configured. However, APP_DOMAIN is also used for the session cookie, which could be relevant when a reverse proxy setup is used.

I'd suggest that the use of APP_DOMAIN for the session cookie is reflected in the documentation.