chore: clarify TransitionConfigurationExchanged event (#9647)

crates/consensus/beacon/src/engine/handle.rs

@@ -87,6 +87,9 @@ where
     /// Sends a transition configuration exchange message to the beacon consensus engine.
     ///
     /// See also <https://github.com/ethereum/execution-apis/blob/3d627c95a4d3510a8187dd02e0250ecb4331d27e/src/engine/paris.md#engine_exchangetransitionconfigurationv1>
+    ///
+    /// This only notifies about the exchange. The actual exchange is done by the engine API impl
+    /// itself.
     pub fn transition_configuration_exchanged(&self) {
         let _ = self.to_engine.send(BeaconEngineMessage::TransitionConfigurationExchanged);
     }

crates/engine/tree/src/tree/mod.rs

@@ -388,7 +388,8 @@ where
                         }
                     }
                     BeaconEngineMessage::TransitionConfigurationExchanged => {
-                        todo!()
+                        // this is a reporting no-op because the engine API impl does not need
+                        // additional input to handle this request
                     }
                 },
                 FromEngine::DownloadedBlocks(blocks) => {