docs: update ZMessage references after blanket impl refactor

Remove misleading 'implement ZMessage' language from actions.md and
protobuf_demo — users no longer write impl ZMessage, it's automatic.

Author: YuanYuYuan

book/src/chapters/actions.md

@@ -336,7 +336,7 @@ impl ZAction for CountAction {
 
 **Key points:**
 
-- `Goal`, `Feedback`, and `Result` can be any struct that implements `Serialize + Deserialize + Clone + Send + Sync + 'static` (blanket implementation via `ZMessage`)
+- `Goal`, `Feedback`, and `Result` can be any struct that is `Send + Sync + 'static` — the `ZMessage` trait is satisfied automatically via a blanket impl, no manual `impl ZMessage` needed
 - `name()` sets the Zenoh key prefix for the action's internal services and topics
 - The default `send_goal_type_info()`, `get_result_type_info()`, etc. all return `TypeHash::zero()`, which is correct for ros-z-to-ros-z communication. For ROS 2 interop, override these with the correct RIHS01 hashes.
 

crates/ros-z/examples/protobuf_demo/src/lib.rs

@@ -140,7 +140,9 @@ pub fn run_service_client(
     println!("Successfully demonstrated protobuf service calls!");
     println!("\nKey points:");
     println!("1. Request/Response: Protobuf messages generated from .proto files");
-    println!("2. Both implement MessageTypeInfo, WithTypeInfo, and ZMessage traits");
+    println!(
+        "2. Both implement MessageTypeInfo and WithTypeInfo (ZMessage is automatic via blanket impl)"
+    );
     println!("3. Service type implements ZService and ServiceTypeInfo");
     println!("4. Works seamlessly with node.create_service() and node.create_client()");
     println!("5. Uses ProtobufSerdes for pure protobuf serialization (not CDR!)");