Document behavior for local addresses in [session_server] configuration
What does this MR do?
This MR documents the behavior in gitlab#30376 (comment 1145596528), noting that local addresses can only be used if the allow_local_requests_from_web_hooks_and_services
application setting is enabled.
Why was this MR needed?
So that users are not confused if they try to use local addresses without the proper configuration choice.