Add missing documentation for sigs-ready event to splice_channel

`ChannelManager::splice_channel` initiates a splice which
ultimately generates a series of events. The most important of
which, `FundingTransactionReadyForSigning` (which must always be
handled, unlike the others), was not documented.

Here we mention the event generation.

Backport 0571f93

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4683,6 +4683,10 @@ where
 	/// emitted. At this point, any inputs contributed to the splice can only be re-spent if an
 	/// [`Event::DiscardFunding`] is seen.
 	///
+	/// After initial signatures have been exchanged, if we contributed any inputs,
+	/// [`Event::FundingTransactionReadyForSigning`] will be generated and
+	/// [`ChannelManager::funding_transaction_signed`] should be called.
+	///
 	/// If any failures occur while negotiating the funding transaction, an [`Event::SpliceFailed`]
 	/// will be emitted. Any contributed inputs no longer used will be included here and thus can
 	/// be re-spent.