Provide guidance on ChannelMonitorUpdate serialized size

Users need to make decisions about storage sizing and we need to
have advice on the maximum size of various things users need to
store. ChannelMonitorUpdates are likely the worst case of this,
they're usually at max a few KB, but can get up to a few hundred
KB for commitment transactions that have 400+ HTLCs pending.

We had one user report an update (likely) going over 400 KiB, which
isn't immediately obvious to me is practical, but its within a few
multiples of trivially-reachable sizes, so its likely that did
occur. To be on the safe side, we simply recommend users ensure
they can support "upwards of 1 MiB" here.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -59,6 +59,11 @@ use sync::Mutex;
 
 /// An update generated by the underlying Channel itself which contains some new information the
 /// ChannelMonitor should be made aware of.
+///
+/// Because these represent only a small number of updates to the underlying state, they are
+/// 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]