Improve receiver name underscore message. (#314)

Remind users that they can omit a receiver name entirely instead of
using an underscore for an unused receiver argument.
Fixes #310.
diff --git a/lint.go b/lint.go
index 935ac83..e1a5f30 100644
--- a/lint.go
+++ b/lint.go
@@ -1238,7 +1238,7 @@
 		name := names[0].Name
 		const ref = styleGuideBase + "#receiver-names"
 		if name == "_" {
-			f.errorf(n, 1, link(ref), category("naming"), `receiver name should not be an underscore`)
+			f.errorf(n, 1, link(ref), category("naming"), `receiver name should not be an underscore, omit the name if it is unused`)
 			return true
 		}
 		if name == "this" || name == "self" {