math/big: add comment about internal assumptions on nat values
math/big: add comment about internal assumptions on nat values

Change-Id: I7ed40507a019c0bf521ba748fc22c03d74bb17b7
Reviewed-on: https://go-review.googlesource.com/100719
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,Cofyc/go,josharian/go,josharian/go,nawawi/go,CAFxX/go,Cofyc/go,codestation/go,josharian/go,CAFxX/go,nawawi/go,codestation/go,cloudflare/go,golang/go,golang/go,josharian/go,codestation/go,Cofyc/go,akutz/go,nawawi/go,Cofyc/go,cloudflare/go,akutz/go,nawawi/go,golang/go,CAFxX/go,codestation/go,codestation/go,Cofyc/go,akutz/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,Cofyc/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,josharian/go,josharian/go,akutz/go,golang/go,codestation/go,golang/go,cloudflare/go,codestation/go,Cofyc/go,Cofyc/go,nawawi/go,josharian/go,codestation/go,cloudflare/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go