Doc: corrections, spelling, and ListView requirements comments

From the ListView comment I thought the model had to be derived from
QAbstractListModel, but that's not the case, QAbstractListModel will
work just fine, it just doesn't support specific cases such as trees.

Change-Id: I59305006540ddc049e276cf412571373cbfa6c67
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>