Make documentation clearer that the top-level elements of a 2D rectangular array are considered **rows** of matrices.

I think it would be very helpful if the docs for MathJS mentioned that `math.multiply` on matrices was pre-multiplication, rather than post-multiplication.

Especially given that all the language at https://mathjs.org/docs/reference/functions/multiply.html strongly implies that `math.multiply(a, b)` is the product `ab` for the rest of the types. The only indication that it's pre-multiplication for matrices is somewhat buried down in the example, where it's only implicit in the dimensions of the input matrices.

Thanks! :)