Add update_persisted_channel doc to indicate rare case for None update

diff --git a/lightning/src/chain/chainmonitor.rs b/lightning/src/chain/chainmonitor.rs
index 086a848af77aa05c7ea4e8e97e93c4e6fb5e9628..d70ebd40d667ad840c012197484d4f94d1f2ced4 100644 (file)
--- a/lightning/src/chain/chainmonitor.rs
+++ b/lightning/src/chain/chainmonitor.rs
@@ -164,8 +164,8 @@ pub trait Persist<ChannelSigner: WriteableEcdsaChannelSigner> {
        /// updated monitor itself to disk/backups. See the [`Persist`] trait documentation for more
        /// details.
        ///
-       /// During blockchain synchronization operations, this may be called with no
-       /// [`ChannelMonitorUpdate`], in which case the full [`ChannelMonitor`] needs to be persisted.
+       /// During blockchain synchronization operations, and in some rare cases, this may be called with
+       /// no [`ChannelMonitorUpdate`], in which case the full [`ChannelMonitor`] needs to be persisted.
        /// Note that after the full [`ChannelMonitor`] is persisted any previous
        /// [`ChannelMonitorUpdate`]s which were persisted should be discarded - they can no longer be
        /// applied to the persisted [`ChannelMonitor`] as they were already applied.