big: more explicit documentation for div/mod/quo/rem
big: more explicit documentation for div/mod/quo/rem

Fixes issue 2380.

R=rsc, rogpeppe
CC=golang-dev
http://codereview.appspot.com/5313041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go