Specified the meaning of unsolvable in setCell.

Author: timlg07

Sudoku-Solver/src/sudoku/model/SudokuBoard.java

@@ -42,6 +42,10 @@ public SudokuBoard(int boxRows, int boxCols) {
      * <p>Setting a cells content to a number removes this number from the 
      * possible values in all other cells sharing a structure with the specified
      * cell.</p>
+     * <p>A sudoku is seen as unsolvable if and only if a structure contains a
+     * duplicate or a cell that can not be set to any number without causing a 
+     * duplicate. It does not check if every structure can contain all numbers.
+     * </p>
      * 
      * @throws IllegalStateException Trying to overwrite a fixed cell.
      */