src/testing/cover: document the CoverBlock struct fields

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>

src/testing/cover.go

@@ -16,11 +16,11 @@ import (
 // NOTE: This struct is internal to the testing infrastructure and may change.
 // It is not covered (yet) by the Go 1 compatibility guidelines.
 type CoverBlock struct {
-	Line0 uint32
-	Col0  uint16
-	Line1 uint32
-	Col1  uint16
-	Stmts uint16
+	Line0 uint32 // line number for block start
+	Col0  uint16 // column number for block start
+	Line1 uint32 // line number for block end
+	Col1  uint16 // column number for block end
+	Stmts uint16 // number of statements included in this block
 }
 
 var cover Cover