Fill in the missing descriptions for the CoverBlock struct fields

Change-Id: I9257881a19b01e5cfe61cf19a91375b6d7cc68ef
GitHub-Last-Rev: f5b9e1d49d1c00f59ce4d3684915e5e93ec0297a
GitHub-Pull-Request: golang/go#24079
Reviewed-on: https://go-review.googlesource.com/96756Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>