message: Update matrix_data documentation
diff: @@ -161,12 +161,18 @@ public:
  /**
  - * This structure is used as an intermediate data type to store the matrix
  - * read from a file in COO-like format.
  + * This structure is used as an intermediate data type to store a sparse matrix.
  *
  - * Note that the structure is not optimized for usual access patterns, can only
  - * exist on the CPU, and thus should only be used for reading matrices from MTX
  - * format.
  + * The matrix is stored as a sequence of nonzero elements, where each element is
  + * a triple of the form (row_index, column_index, value).
  + *
  + * @note All Ginkgo functions returning such a structure will return the
  + * nonzeros sorted in row-major order.
  + * @note All Ginkgo functions that take this structure as input expect that the
  + * nonzeros are sorted in row-major order.
  + * @note This structure is not optimized for usual access patterns and it can
  + * only exist on the CPU. Thus, it should only be used for utility
  + * functions which do not have to be optimized for performance.
  *
  * @tparam ValueType type of matrix values stored in the structure
  * @tparam IndexType type of matrix indexes stored in the structure
  @@ -199,7 +205,7 @@ struct matrix_data {
  std::vector<nonzero_type> nonzeros;
  /**
  - * Sorts the nonzero list in row-major order.
  + * Sorts the nonzero vector in row-major order.
  */
  void sort()
  {
  