message: Clarify that SparseMatMul does not accept SparseTensor inputs
diff: @@ -3,9 +3,11 @@ op {
  summary: "Multiply matrix \"a\" by matrix \"b\"."
  description: <<END
  The inputs must be two-dimensional matrices and the inner dimension of "a" must
  -match the outer dimension of "b". This op is optimized for the case where at
  -least one of "a" or "b" is sparse. The breakeven for using this versus a dense
  -matrix multiply on one platform was 30% zero values in the sparse matrix.
  +match the outer dimension of "b". Both "a" and "b" must be `Tensor`s not
  +`SparseTensor`s. This op is optimized for the case where at least one of "a" or
  +"b" is sparse, in the sense that they have a large proportion of zero values.
  +The breakeven for using this versus a dense matrix multiply on one platform was
  +30% zero values in the sparse matrix.
  The gradient computation of this operation will only take advantage of sparsity
  in the input gradient when that gradient comes from a Relu.
  