Document vtkTypedDataArray<vtkIdType>::GetDataArray() behavior.

Add some notes explaining that an extra step must be taken when
subclasses vtkTypedDataArray to hold vtkIdType elements.

Change-Id: I0063c97529e5dd15713186184920c6100ab5452c
Merge topic 'vtkIdType-mapped-array-doc' into master

c4cd8246 Document vtkTypedDataArray<vtkIdType>::GetDataArray() behavior.