doc: Clarify that DelegateModel is a kind of proxy model

It was not quite clear in the GridView DnD example that the ListModel
doesn't get reordered, so if new ListElements are inserted after reordering
the DelegateModel by DnD, predicting the position where new delegates will
appear becomes a bit of a riddle.

Also:
- the QQmlDelegateModelAttached::model is a model, not an int
- disambiguate the properties that have the same names in
  QQmlDelegateModel and QQmlDelegateModelAttached, using \keyword
  for linking.

Task-number: QTBUG-34891
Change-Id: I485fd632f67d607652428b4e3c9ca528e57f7348
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

0 files changed, 0 insertions, 0 deletions