message: PetscParMatrix: clarify constructor
diff: @@ -238,7 +238,9 @@ public:
  @param[in] ref If true, we increase the reference count of @a a. */
  PetscParMatrix(Mat a, bool ref=false);
  - /** @brief Convert a PetscParMatrix @a pa with a new PETSc format @a tid. */
  + /** @brief Convert a PetscParMatrix @a pa with a new PETSc format @a tid.
  + Note that if @a pa is already a PetscParMatrix of the same type as
  + @a tid, the resulting PetscParMatrix will share the same Mat object */
  explicit PetscParMatrix(const PetscParMatrix *pa, Operator::Type tid);
  /** @brief Creates a PetscParMatrix extracting the submatrix of @a A with
  