Use textAt(index) in ComboBox delegate implementations and documentation

See discussion here:

https://codereview.qt-project.org/c/qt/qtdeclarative/+/419764/3..4/src/quickcontrols2/doc/snippets/qtquickcontrols2-combobox-custom.qml#b17

This greatly simplifies the expression, at the expense of a very uncommon use case that can still be achieved by the user providing their own delegate implementation (and the documentation that we will provide them).