nixos/*: convert straggler options to MD

nixos/modules/services/web-servers/apache-httpd/vhost-options.nix

@@ -116,12 +116,12 @@ in
     useACMEHost = mkOption {
       type = types.nullOr types.str;
       default = null;
-      description = ''
+      description = lib.mdDoc ''
         A host of an existing Let's Encrypt certificate to use.
         This is useful if you have many subdomains and want to avoid hitting the
-        <link xlink:href="https://letsencrypt.org/docs/rate-limits/">rate limit</link>.
-        Alternately, you can generate a certificate through <option>enableACME</option>.
-        <emphasis>Note that this option does not create any certificates, nor it does add subdomains to existing ones – you will need to create them manually using  <xref linkend="opt-security.acme.certs"/>.</emphasis>
+        [rate limit](https://letsencrypt.org/docs/rate-limits).
+        Alternately, you can generate a certificate through {option}`enableACME`.
+        *Note that this option does not create any certificates, nor it does add subdomains to existing ones – you will need to create them manually using [](#opt-security.acme.certs).*
       '';
     };