Replace doc with deprecated attribute

Author: emhane

Diff for: crates/consensus/beacon/src/engine/mod.rs

@@ -167,8 +167,7 @@ type PendingForkchoiceUpdate<PayloadAttributes> =
 /// # Panics
 ///
 /// If the future is polled more than once. Leads to undefined state.
-///
-/// Note: soon deprecated. See `reth_engine_service::EngineService`.
+#[deprecated(note = "Replaced by `reth_engine_service::EngineService`, type will soon be removed.")]
 #[must_use = "Future does nothing unless polled"]
 #[allow(missing_debug_implementations)]
 pub struct BeaconConsensusEngine<N, BT, Client>