update_agent: reword reboot delay broadcast mesage

Reword message broadcasted to users when reboots are delayed due
to active interactive user sessions to emphasize that logging out
active sessions will allow Zincat to continue the auto-update
process.

src/update_agent/mod.rs

@@ -473,7 +473,10 @@ fn format_reboot_warning(seconds: u64, release_ver: &str) -> String {
     let time_till_reboot = format_seconds(seconds);
 
     format!(
-        "New update {} deployed.\nRebooting into this update in around {} (if permitted by update strategy).",
+        "New update {} is available and has been deployed.\n\
+        If permitted by the update strategy, Zincati will reboot into this update when all\n\
+        interactive users have logged out, or in {}, whichever comes earlier.\n\
+        Please log out of all active sessions in order to let the auto-update process continue.",
         release_ver, time_till_reboot
     )
 }