BUG #14873: Fixed selections on vtkTable datasets in spreadsheet.

Spreadsheet view didn't show rows selected in vtkTable properly due to
missing logic in vtkMarkSelectedRows to mark selection rows. Fixed that.

Also update documentation in pqSpreadSheetViewSelectionModel to better
explain its role.

Change-Id: I8a1b26f0a193a6bd4be8f0c3e62f6727a09c0e31
Merge topic '14873_fix_table_selections' into next

8e345d4d BUG #14873: Fixed selections on vtkTable datasets in spreadsheet.