go spec: clarify semantics of integer division
go spec: clarify semantics of integer division

Fixes issue 1764.

R=rsc, r, iant, ken2
CC=golang-dev
http://codereview.appspot.com/4431082

tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,ssrl/go,apparentlymart/golang,ssrl/go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,ssrl/go,gnanderson/go,dsriram/golang,tv42/old-go,ssrl/go,gnanderson/go,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,ssrl/go,dsriram/golang,ssrl/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,ssrl/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme