math/big: comment division

The comments in the code refer to Knuth and to Burnikel and Ziegler,
but Knuth's presentation is inscrutable, and our recursive division
code does not bear much resemblance to Burnikel and Ziegler's paper
(which is fine, ours is nicer).

Add a standalone explanation of division instead of referring to
difficult or not-directly-used references.

Change-Id: Ic1b35dc167fb29a69ee00e0b4a768ac9cc9e1324
Reviewed-on: https://go-review.googlesource.com/c/go/+/321078
Trust: Russ Cox <rsc@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>