Clarify splicing feature flag requirements

Author: tankyleo

CHANGELOG.md

@@ -7,7 +7,11 @@
    signaling in a future version as testing completes, breaking compatibility.
    Support for accepting splices is gated on
    `UserConfig::reject_inbound_splices`. Outbound splices can be initiated with
-   `ChannelManager::splice_channel`.
+   `ChannelManager::splice_channel`. Initiating a splice requires that the
+   channel counterparty supports splicing. Existing channels can be spliced, as
+   long as a reconnection with the counterparty occurred after they enabled
+   splicing. All channel types can be spliced, namely legacy, anchor, and
+   zero-fee-commitment channels.
  * Various APIs have been updated to offer a native Rust async API. All
    newly-async traits and structs have a `*Sync` variant which offers the
    same API but with sync methods:

lightning/src/ln/channelmanager.rs

@@ -4707,6 +4707,13 @@ 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. Existing
+	/// channels can be spliced, as long as a reconnection with the counterparty occurred after they
+	/// enabled splicing. All channel types can be spliced, namely legacy, anchor, and
+	/// zero-fee-commitment channels.
+	///
 	/// # Arguments
 	///
 	/// Provide a `contribution` to determine if value is spliced in or out. The splice initiator is