Typos/Wrong Info in doc/src/examples/editabletreemodel.qdoc

Section Overview.

a) Existing text is: 'As described in the Model Subclassing Reference, models must provide implementations for the standard set of model functions: flags(), data(), headerData(), and rowCount().'

Missing/Should be: Models also must provide an implementation for colCount()

b) Next section: "An editable model needs to provide implementations of setData() and headerData()"

Missing/Should be: An editable model needs to provide implementations of setData() and setHeaderData(),