Clarify RBF strategy for channel opens

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1539,6 +1539,10 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// Do NOT broadcast the funding transaction yourself. When we have safely received our
 	/// counterparty's signature the funding transaction will automatically be broadcast via the
 	/// [`BroadcasterInterface`] provided when this `ChannelManager` was constructed.
+	///
+	/// Note that this includes RBF or similar transaction replacement strategies - lightning does
+	/// not currently support replacing a funding transaction on an existing channel. Instead,
+	/// create a new channel with a conflicting funding transaction.
 	pub fn funding_transaction_generated(&self, temporary_channel_id: &[u8; 32], funding_transaction: Transaction) -> Result<(), APIError> {
 		let _persistence_guard = PersistenceNotifierGuard::new(&self.total_consistency_lock, &self.persistence_notifier);