People always want to remove concatstring{2,3,4,5} for performance,
but we keep them to make the binary smaller. So, add a comment to
document why.
Updates #65020
Change-Id: I819976b700d45ce4d0846bf4481b2654b85708da
Reviewed-on: https://go-review.googlesource.com/c/go/+/700095
Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>