Update doc

include/ginkgo/core/preconditioner/jacobi.hpp

@@ -200,6 +200,10 @@ struct block_interleaved_storage_scheme {
  * @note When using the adaptive variant, there may be a trade-off in terms of
  * slightly longer preconditioner generation due to extra work required to
  * detect the optimal precision of the blocks.
+ * @note When the max_block_size is set to 1, specialized kernels are used,
+ * both for generation (inverting the diagonals) and application (diagonal
+ * scaling) to reduce the overhead involved in the usual (adaptive) block
+ * case.
  *
  * @ingroup jacobi
  * @ingroup precond
@@ -300,7 +304,10 @@ class Jacobi : public EnableLinOp<Jacobi<ValueType, IndexType>>,
  /**
  * Maximal size of diagonal blocks.
  *
- * @note This value has to be between 1 and 32 (NVIDIA)/64 (AMD).
+ * @note This value has to be between 1 and 32 (NVIDIA)/64 (AMD). For
+ * efficiency, when the max_block_size is set to 1, specialized kernels
+ * are used and the additional objects (block_ptrs etc) are set to null
+ * values.
  */
  uint32 GKO_FACTORY_PARAMETER_SCALAR(max_block_size, 32u);