[Rd] M[cbind()] <- assignment with Matrix object loses attributes

[Abby Spurdle]

> Hmm, really?  In `R Under development (unstable) (2020-08-14
> r79020)`, doing the indexed assignment with a regular matrix (as opposed
> to a Matrix) appears to preserve attributes.

I was referring to *Matrix* objects.
Sorry, if that wasn't clear.