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/services/web-servers/nginx/default.nix

@@ -385,7 +385,7 @@ in
 {
   options = {
     services.nginx = {
-      enable = mkEnableOption "Nginx Web Server";
+      enable = mkEnableOption (lib.mdDoc "Nginx Web Server");
 
       statusPage = mkOption {
         default = false;