Channel monitor syncing in Electrum backend example

[ConorOkus]

https://lightningdevkit.org/tutorials/build_a_node_in_rust/#10-sync-channelmonitors-and-channelmanager-to-chain-tip

Add a note to make sure channel monitors are in sync first before doing it for channel manager and chain monitor.

Can also add an extra step around populating the filter when instantiating a chain monitor

• ChannelMonitor::load_outputs_to_watch