The function documentation was wrong, it was using a wrong parameter. This change
replaces it with the right parameter.

The wrong formula was: q = (u1<<_W + u0 - r)/y
The function has got a parameter "v" (of type Word), not a parameter "y".
So, the right formula is: q = (u1<<_W + u0 - r)/v

Fixes #28444

Change-Id: I82e57ba014735a9fdb6262874ddf498754d30d33
Reviewed-on: https://go-review.googlesource.com/c/145280Reviewed-by: Robert Griesemer <gri@golang.org>