Clarify nodeset vs ref attributes

Our spec implies that `ref `and `nodeset` can be used interchangeably in the `` and just recommends to use nodeset for `` and `` elements. I think this is not quite correct. I believe we **have to** use `` where allowed and where we refer to (possibly) multiple nodes (e.g. multiple repeat instances). So a [``](https://www.w3.org/TR/xforms11/#ui-repeat-module) and [``](https://www.w3.org/TR/xforms11/#ui-selection-commonelems-itemset) **have to have** nodeset attributes. (P.S. unhelpfully [Orbeon uses refs for repeats](https://doc.orbeon.com/xforms/actions/repeat-insert-delete). [Update] [This is why](https://blog.orbeon.com/2012/04/xforms-20-goodbye-nodeset.html) (XForms 2.0): ).

Start by reviewing the specification wording for ref and nodeset in the , , and sections, along with the linked XForms repeat and itemset references. Clarify whether nodeset is required when multiple nodes may be addressed, and update the relevant specification text so the attribute rules and rationale are explicit.

Written by the indexing model from the issue text.