spec: Update specification for Set structure

* Inside a Set object, label is no longer optional
* The "sets" container (Root.sets) will be an array instead
  of an object. They don't need to have an internal key.
  Contrary to Root.params where the key relates to the
  parameter key in wikitext.

Change-Id: Iede29d25cd6ecbcf43970e1e45e60e5aac206a95