Structure: Document ID required

Certain nodes within the Structure tool require a document ID to operate on. The Studio reports document id (`id`) is required for document nodes when a document node is given an empty or undefined document ID. If you omit the ID entirely, the pane ID is used instead and you get `id` is required for document nodes instead.

Setting a document ID can be done by calling the documentId() method:

S.document()
  .id('car-editor')
  .schemaType('car')
  .documentId('am-db9')