message: clarify comment for skipping zeros
diff: @@ -2471,8 +2471,9 @@ void SparseMatrix::AddSubMatrix(const Array<int> &rows, const Array<int> &cols,
  a = subm(i, j);
  if (skip_zeros && a == 0.0)
  {
  - // if the element is zero do not assemble it unless this breaks
  - // the symmetric structure
  + // Skip assembly of zero elements if either:
  + // (i) user specified to skip zeros regardless of symmetry, or
  + // (ii) symmetry is not broken.
  if (skip_zeros == 2 || &rows != &cols || subm(j, i) == 0.0)
  {
  continue;
  