SIGHUP for reloading configuration
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 SIGHUPand from there, a longer description with implementation example
In my experience, SIGHUP is commonly used to request that a daemon reload its configuration. Using this well-known technique follows the principle of least surprise. Your users have likely encountered daemons which do exactly this, so they'll immediately understand what SIGHUP means to your daemon.