message: document skip_zeros behavior for SparseMatrix::AddSubMatrix()
diff: @@ -523,6 +523,12 @@ public:
  void SetSubMatrixTranspose(const Array<int> &rows, const Array<int> &cols,
  const DenseMatrix &subm, int skip_zeros = 1);
  + /** Insert the DenseMatrix into this SparseMatrix at the specified rows and
  + columns. If \c skip_zeros==0 , all entries from the DenseMatrix are
  + added including zeros. If \c skip_zeros==2 , no zeros are added to the
  + SparseMatrix regardless of their position in the matrix. Otherwise, the
  + default \c skip_zeros behavior is to omit the zero from the SparseMatrix
  + unless it would break the symmetric structure of the SparseMatrix. */
  void AddSubMatrix(const Array<int> &rows, const Array<int> &cols,
  const DenseMatrix &subm, int skip_zeros = 1);
  