Add more details about async persistence completion/backgrounding

This clarifies somewhat that async persistence should run
indefinitely or keep trying via polling, and that either is
acceptable.

Author: TheBlueMatt

lightning/src/chain/chainmonitor.rs

@@ -113,7 +113,10 @@ impl MonitorUpdateId {
 ///  [`ChainMonitor::channel_monitor_updated`] must be called once for *each* update which occurs.
 ///
 ///  If at some point no further progress can be made towards persisting a pending update, the node
-///  should simply shut down.
+///  should simply shut down. Until then, the background task should either loop indefinitely, or
+///  persistence should be regularly retried with [`ChainMonitor::list_pending_monitor_updates`]
+///  and [`ChainMonitor::get_monitor`] (note that if a full monitor is persisted all pending
+///  monitor updates may be marked completed).
 ///
 /// # Using remote watchtowers
 ///