In http://jsonapi.org/format/#crud-creating, the spec says,

If a relationship is provided in the relationships member of the resource object, its value MUST be a relationship object with a data member. The value of this key represents the linkage the new resource is to have.

The use of "the linkage" suggests that there can be only one relationship of a particular type.

So while

{
  "relationships": {
    "tags": {
      "data": [
        { "type": "tag", "id": 12345 },
        { "type": "tag", "id": 67890 }
      ]
    }
  }
}

is specifically allowed in fetching, it doesn't seem to be for creating.

Suggestions:

• include a link to the relationships object docs in this section
• state that any structure allowed in fetching is allowed in creating, so long as each resource linkage contains at minimum, { id, type }.
• change "the linkage" to "the linkage(s)"