nixos: remove all uses of lib.mdDoc

these changes were generated with nixq 0.0.2, by running

  nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix

two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.

Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.

nixos/modules/programs/command-not-found/command-not-found.nix

@@ -26,7 +26,7 @@ in
     enable = mkOption {
       type = types.bool;
       default = true;
-      description = lib.mdDoc ''
+      description = ''
         Whether interactive shells should show which Nix package (if
         any) provides a missing command.
       '';
@@ -34,7 +34,7 @@ in
 
     dbPath = mkOption {
       default = "/nix/var/nix/profiles/per-user/root/channels/nixos/programs.sqlite" ;
-      description = lib.mdDoc ''
+      description = ''
         Absolute path to programs.sqlite.
 
         By default this file will be provided by your channel