nixos/*: automatically convert option descriptions

conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running

    nix-doc-munge nixos/**/*.nix
    nix-doc-munge --import nixos/**/*.nix

the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.

nixos/modules/programs/npm.nix

@@ -11,7 +11,7 @@ in
 
   options = {
     programs.npm = {
-      enable = mkEnableOption "<command>npm</command> global config";
+      enable = mkEnableOption (lib.mdDoc "{command}`npm` global config");
 
       package = mkOption {
         type = types.package;