nixos: Explain system.build.installBootLoader's odd default

I don't really approve of this solution, but documenting its
purpose was the least I could do for now.

nixos/modules/system/activation/top-level.nix

@@ -227,6 +227,9 @@ in
     system.build = tmpFixupSubmoduleBoundary {
       installBootLoader = mkOption {
         internal = true;
+        # "; true" => make the `$out` argument from switch-to-configuration.pl
+        #             go to `true` instead of `echo`, hiding the useless path
+        #             from the log.
         default = "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
         description = ''
           A program that writes a bootloader installation script to the path passed in the first command line argument.