math/big: add a comment for SetMantExp
math/big: add a comment for SetMantExp

Change-Id: I9ff5d1767cf70648c2251268e5e815944a7cb371
Reviewed-on: https://go-review.googlesource.com/c/go/+/233737
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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