Label a document

Adds and/or removes labels on one document — the record itself is never changed, so this is allowed on a document whose versions are sealed. Name each label by its id (from documents_list_labels) or by its exact name; a name that two labels share (the same «2026» under two parents) is refused with the candidates, so pass the id then. Labels must already exist — documents_create_label makes one. Answers with the document’s labels as they now stand.