a root menu document can not be added anywhere in the tree, only at the root

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | see travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

as the menu document is supposed to be a menu root, no need to select the parent, it is automatically defined.