[mlir] Fix Tensor_InsertSliceOp description

This commit fixes `Tensor_InsertSliceOp` `sizes` inputs/attributes
description.

Before this commit, the description says the `sizes` inputs/attributes
denote the size of the return type. But according to the
`InsertSliceOpConstantArgumentFolder` in
`lib/Dialect/Tensor/IR/TensorOps.cpp`, the `sizes` inputs/attributes
actually denote the size of the source type.

I had an off-line discussion with the authors of `TensorOps.td` and
`TensorOps.cpp`. We concluded that it was a typo in the Op description.

This commit updates the Op description to match the actual usage.

Differential Revision: https://reviews.llvm.org/D126264