nixos/nextcloud: extend documentation for nginx configuration

Co-authored-by: Robin Gloster <mail@glob.in>

nixos/modules/services/web-apps/nextcloud.nix

@@ -70,7 +70,15 @@ in {
       '';
     };
 
-    nginx.enable = mkEnableOption "nginx vhost management";
+    nginx.enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enable nginx virtual host management.
+        Further nginx configuration can be done by adapting <literal>services.nginx.virtualHosts.&lt;name&gt;</literal>.
+        See <xref linkend="opt-services.nginx.virtualHosts"/> for further information.
+      '';
+    };
 
     webfinger = mkOption {
       type = types.bool;