cmd/go: fix ugly "BUG" in go help message output

Change-Id: I4c579d0d02f6c9cbef1dc2bc2c93cff614538dea
Reviewed-on: https://go-review.googlesource.com/44854
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -1436,9 +1436,10 @@
 //
 // 	-cover
 // 	    Enable coverage analysis.
-//
-// 	    BUG: If a compilation or test fails with coverage enabled,
-// 	    the reported line numbers may be incorrect.
+// 	    Note that because coverage works by annotating the source
+// 	    code before compilation, compilation and test failures with
+// 	    coverage enabled may report line numbers that don't correspond
+// 	    to the original sources.
 //
 // 	-covermode set,count,atomic
 // 	    Set the mode for coverage analysis for the package[s]

src/cmd/go/internal/test/test.go

@@ -161,9 +161,10 @@ const testFlag2 = `
 
 	-cover
 	    Enable coverage analysis.
-
-	    BUG: If a compilation or test fails with coverage enabled,
-	    the reported line numbers may be incorrect.
+	    Note that because coverage works by annotating the source
+	    code before compilation, compilation and test failures with
+	    coverage enabled may report line numbers that don't correspond
+	    to the original sources.
 
 	-covermode set,count,atomic
 	    Set the mode for coverage analysis for the package[s]