spec: mention that special case for integer division is due to overflow
spec: mention that special case for integer division is due to overflow

Fixes #23443.

Change-Id: If60c39b582ee5308e9fa902f93c1b6ae7890346c
Reviewed-on: https://go-review.googlesource.com/87975
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

cloudflare/go,codestation/go,josharian/go,Cofyc/go,Cofyc/go,akutz/go,josharian/go,josharian/go,codestation/go,codestation/go,cloudflare/go,cloudflare/go,christopher-henderson/Go,nawawi/go,codestation/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,Cofyc/go,codestation/go,golang/go,Cofyc/go,golang/go,akutz/go,nawawi/go,golang/go,josharian/go,golang/go,CAFxX/go,akutz/go,akutz/go,akutz/go,Cofyc/go,Cofyc/go,codestation/go,Cofyc/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,codestation/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,Cofyc/go,josharian/go,nawawi/go,codestation/go,nawawi/go,akutz/go,josharian/go,golang/go,nawawi/go,golang/go,cloudflare/go