testing/cover: improve comments on CoverBlock

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>

src/testing/cover.go

@@ -13,14 +13,17 @@ import (
 )
 
 // CoverBlock records the coverage data for a single basic block.
+// The fields are 1-indexed, as in an editor: The opening line of
+// the file is number 1, for example. Columns are measured
+// in bytes.
 // 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 // 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
+	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