While package comments shouldn't be novels, this throwaway word was not sufficient (and wasn't mirrored in the `box` package). This change attempts to include more reasoning without using too many words. Fixes golang/go#17673,golang/go#21139 Change-Id: I7fa11e2cd5b8e2010420cc14d784f9b0c65db6d2 Reviewed-on: https://go-review.googlesource.com/35910 Reviewed-by: Russ Cox <rsc@golang.org>