math/big: document that Rat.Denom might modify the receiver
math/big: document that Rat.Denom might modify the receiver

Fixes #33792

Change-Id: I306a95883c3db2d674d3294a6feb50adc50ee5d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/192017
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,golang/go,akutz/go,golang/go,cloudflare/go,akutz/go