fix documentation links

PiperOrigin-RevId: 707519488
Change-Id: Ib93014e51ed841f6ba08244873c094b1c438760f
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/637395
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
diff --git a/open2opaque.go b/open2opaque.go
index bbc9444..a41895c 100644
--- a/open2opaque.go
+++ b/open2opaque.go
@@ -37,8 +37,15 @@
 	// Prepend general documentation before the regular help output.
 	defaultExplain := commander.Explain
 	commander.Explain = func(w io.Writer) {
-		fmt.Fprintf(w, "The open2opaque tool migrates Go packages from the open protobuf API to the opaque protobuf API.\n\n")
-		fmt.Fprintf(w, "See http://godoc/3/third_party/golang/google_golang_org/open2opaque/v/v0/open2opaque for documentation.\n\n")
+		fmt.Fprintf(w, `The open2opaque tool migrates Go packages from the Go Protobuf Open Struct API to the Opaque API.
+
+For documentation, see:
+* https://go.dev/blog/protobuf-opaque
+* https://protobuf.dev/reference/go/opaque-migration/
+
+Report issues at https://github.com/golang/open2opaque/issues
+
+`)
 		defaultExplain(w)
 	}
 
@@ -51,7 +58,7 @@
 	const groupRewrite = "automatically rewriting Go code"
 	commander.Register(rewrite.Command(), groupRewrite)
 
-	const groupFlag = "managing go_api_flag"
+	const groupFlag = "managing the API level"
 	commander.Register(setapi.Command(), groupFlag)
 
 	flag.Usage = func() {