diff --git a/nixos/modules/system/boot/systemd.nix b/nixos/modules/system/boot/systemd.nix
index ac55461107f..b6c3085c4f1 100644
--- a/nixos/modules/system/boot/systemd.nix
+++ b/nixos/modules/system/boot/systemd.nix
@@ -395,7 +395,9 @@ in
       description = lib.mdDoc ''
         The amount of time which can elapse after a reboot has been triggered
         before a watchdog hardware device will automatically reboot the system.
-        Valid time units include "ms", "s", "min", "h", "d", and "w".
+        Valid time units include "ms", "s", "min", "h", "d", and "w". If left
+        `null`, systemd will use its default of `10min`; see also {command}`man
+        5 systemd-system.conf`.
       '';
     };