SIGHUP as a notification about terminal closing event doesn’t make sense for a daemon, because deamons are detached from their terminal. So the system will never send this signal to them.
Then it is common practice for daemons to use it for another meaning, typically reloading the daemon’s configuration.
This is not a rule, just kind of a convention. That’s why it’s not documented in the manpage.
See the wikipedia entry for SIGHUP
and from there, a longer description with implementation example