Merge pull request #1670 from TheBlueMatt/2022-08-mon-size-guidelines

Provide guidance on ChannelMonitorUpdate serialized size

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -59,8 +59,13 @@ use core::convert::TryInto;
 use core::ops::Deref;
 use sync::Mutex;
 
-/// An update generated by the underlying Channel itself which contains some new information the
-/// ChannelMonitor should be made aware of.
+/// An update generated by the underlying channel itself which contains some new information the
+/// [`ChannelMonitor`] should be made aware of.
+///
+/// Because this represents only a small number of updates to the underlying state, it is generally
+/// much smaller than a full [`ChannelMonitor`]. However, for large single commitment transaction
+/// updates (e.g. ones during which there are hundreds of HTLCs pending on the commitment
+/// transaction), a single update may reach upwards of 1 MiB in serialized size.
 #[cfg_attr(any(test, fuzzing, feature = "_test_utils"), derive(PartialEq))]
 #[derive(Clone)]
 #[must_use]