os: clarify that File.{Read,Write} use the buffer

Fixes #49470

Change-Id: I81fd4b0e2eef1d8d430b5d1d10c4f824e803a75c
Reviewed-on: https://go-review.googlesource.com/c/go/+/362335


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>