Doc that Record::node_id may be missing even with channel_id

There are various place where we log something related to a channel
but fail to fill in the channel's counterparty information. This is
somewhat surprising, given channel counterparty information is
always known, but simply is sometimes not readily accessible to LDK
when a log is printed.

Author: TheBlueMatt

lightning/src/util/logger.rs

@@ -98,6 +98,10 @@ pub struct Record<'a> {
 	/// The verbosity level of the message.
 	pub level: Level,
 	/// The node id of the peer pertaining to the logged record.
+	///
+	/// Note that in some cases a [`Self::channel_id`] may be filled in but this may still be
+	/// `None`, depending on if the peer information is readily available in LDK when the log is
+	/// generated.
 	pub peer_id: Option<PublicKey>,
 	/// The channel id of the channel pertaining to the logged record. May be a temporary id before
 	/// the channel has been funded.