proto: extend Unmarshal documentation, include an example

This example uses the same protobuf and wire format encoding
as the corresponding Marshal example added in commit
https://go.googlesource.com/protobuf/+/c69658e23457d4e09

Change-Id: Ifd64a93a14589595cbe9b218235b57fb15d423c2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/574635
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

proto/decode.go

@@ -51,6 +51,8 @@ type UnmarshalOptions struct {
 
 // Unmarshal parses the wire-format message in b and places the result in m.
 // The provided message must be mutable (e.g., a non-nil pointer to a message).
+//
+// See the [UnmarshalOptions] type if you need more control.
 func Unmarshal(b []byte, m Message) error {
 	_, err := UnmarshalOptions{RecursionLimit: protowire.DefaultRecursionLimit}.unmarshal(b, m.ProtoReflect())
 	return err

proto/decode_test.go

@@ -14,6 +14,7 @@ import (
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/reflect/protoreflect"
 	"google.golang.org/protobuf/testing/protopack"
+	"google.golang.org/protobuf/types/known/durationpb"
 
 	"google.golang.org/protobuf/internal/errors"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
@@ -155,3 +156,20 @@ func extend(desc protoreflect.ExtensionType, value interface{}) buildOpt {
 		proto.SetExtension(m, desc, value)
 	}
 }
+
+// This example illustrates how to unmarshal (decode) wire format encoding into
+// a Protobuf message.
+func ExampleUnmarshal() {
+	// This is the wire format encoding produced by the Marshal example.
+	// Typically you would read from the network, from disk, etc.
+	b := []byte{0x10, 0x7d}
+
+	var dur durationpb.Duration
+	if err := proto.Unmarshal(b, &dur); err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Protobuf wire format decoded to duration %v\n", dur.AsDuration())
+
+	// Output: Protobuf wire format decoded to duration 125ns
+}