The previous CL (https://go-review.googlesource.com/c/go/+/96756)
added comments that didn't really say much, but there is something
so say: what the units are and that they are indexed starting at 1.

Add a more helpful comment on the type, and also follow proper
style by using initial capitals and a period.

Change-Id: Id19cd5f392faf7c7bac034073f276cc770589075
Reviewed-on: https://go-review.googlesource.com/121875Reviewed-by: Ian Lance Taylor <iant@golang.org>