Document SpliceNegotiationFailed contribution / DiscardFunding overlap

The contribution returned in Event::SpliceNegotiationFailed may include
inputs and outputs already committed to a prior negotiated (but not yet
locked) splice transaction. Those overlapping items are intentionally
omitted from the preceding Event::DiscardFunding to avoid prompting the
user to reclaim UTXOs that are still in use elsewhere. The relationship
was documented on the internal SpliceFundingFailed fields but lost when
they were made private; surface it on the public event field doc.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jkczyz

lightning/src/events/mod.rs

@@ -1706,6 +1706,12 @@ pub enum Event {
 		/// Alternatively, call [`ChannelManager::splice_channel`] to obtain a fresh
 		/// [`FundingTemplate`] and build a new contribution.
 		///
+		/// The contribution preserves the full set of inputs and outputs from the failed round,
+		/// including any that were also committed to a prior negotiated (but not yet locked)
+		/// splice transaction. Those overlapping inputs and outputs are intentionally omitted
+		/// from the preceding [`Event::DiscardFunding`], since they remain committed to that
+		/// prior splice.
+		///
 		/// [`ChannelManager::funding_contributed`]: crate::ln::channelmanager::ChannelManager::funding_contributed
 		/// [`ChannelManager::splice_channel`]: crate::ln::channelmanager::ChannelManager::splice_channel
 		/// [`FundingTemplate`]: crate::ln::funding::FundingTemplate