In: https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/system/boot/loader/grub/grub.nix#L116

-        description = ''
-          The device on which the GRUB boot loader will be installed.
-          The special value <literal>nodev</literal> means that a GRUB
-          boot menu will be generated, but GRUB itself will not
-          actually be installed.  To install GRUB on multiple devices,
-          use <literal>boot.loader.grub.devices</literal>.
-        '';

+        description = ''
+          The device on which the GRUB boot loader will be installed.
+          The special value <literal>nodev</literal> means that a GRUB
+          boot menu will be generated, but GRUB itself will not
+          actually be installed.  To install GRUB on multiple devices,
+          use <literal>boot.loader.grub.devices</literal>.
+          Disk serial ids ("/dev/by-id/XXXX") are also accepted.
+       '';