Clarify splicing feature flag requirements

Author: tankyleo

lightning/src/ln/channelmanager.rs

@@ -4854,6 +4854,12 @@ where
 	/// the channel. This will spend the channel's funding transaction output, effectively replacing
 	/// it with a new one.
 	///
+	/// # Required Feature Flags
+	///
+	/// Initiating a splice requires that the channel counterparty supports splicing. Any
+	/// channel (no matter the type) can be spliced, as long as a reconnection with the
+	/// counterparty occurred after they enabled splicing.
+	///
 	/// # Arguments
 	///
 	/// Provide a `contribution` to determine if value is spliced in or out. The splice initiator is