Unify README and program help output

PiperOrigin-RevId: 707525670
Change-Id: I35f1c58ed35661e71163d5693cf588a65c920a7e
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/637357
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Chressie Himpel <chressie@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>

README.md

diff --git a/README.md b/README.md
index 68e2965..d29d93b 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,8 @@
 # open2opaque
 
-The program open2opaque migrates Go code using Go Protobuf from the Open API
+The open2opaque tool migrates Go packages from the Go Protobuf Open Struct API
 to the Opaque API.
 
-See https://go.dev/blog/protobuf-opaque for context.
+For documentation, see:
+* https://go.dev/blog/protobuf-opaque
+* https://protobuf.dev/reference/go/opaque-migration/