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/apache-httpd/default.nix

@@ -404,7 +404,7 @@ in
 
     services.httpd = {
 
-      enable = mkEnableOption "the Apache HTTP Server";
+      enable = mkEnableOption (lib.mdDoc "the Apache HTTP Server");
 
       package = mkOption {
         type = types.package;
@@ -445,11 +445,11 @@ in
             { name = "jk"; path = "''${pkgs.tomcat_connectors}/modules/mod_jk.so"; }
           ]
         '';
-        description = ''
+        description = lib.mdDoc ''
           Additional Apache modules to be used. These can be
           specified as a string in the case of modules distributed
           with Apache, or as an attribute set specifying the
-          <varname>name</varname> and <varname>path</varname> of the
+          {var}`name` and {var}`path` of the
           module.
         '';
       };