f mention async in cm docs

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -905,12 +905,14 @@ where
 /// called [`funding_transaction_generated`] for outbound channels) being closed.
 ///
 /// Note that you can be a bit lazier about writing out `ChannelManager` than you can be with
-/// [`ChannelMonitor`]. With [`ChannelMonitor`] you MUST write each monitor update out to disk before
-/// returning from [`chain::Watch::watch_channel`]/[`update_channel`], with ChannelManagers, writing updates
-/// happens out-of-band (and will prevent any other `ChannelManager` operations from occurring during
-/// the serialization process). If the deserialized version is out-of-date compared to the
-/// [`ChannelMonitor`] passed by reference to [`read`], those channels will be force-closed based on the
-/// `ChannelMonitor` state and no funds will be lost (mod on-chain transaction fees).
+/// [`ChannelMonitor`]. With [`ChannelMonitor`] you MUST durably write each
+/// [`ChannelMonitorUpdate`] before returning from
+/// [`chain::Watch::watch_channel`]/[`update_channel`] or before completing async writes. With
+/// ChannelManagers, writing updates happens out-of-band (and will prevent any other
+/// `ChannelManager` operations from occurring during the serialization process). If the
+/// deserialized version is out-of-date compared to the [`ChannelMonitor`] passed by reference to
+/// [`read`], those channels will be force-closed based on the `ChannelMonitor` state and no funds
+/// will be lost (modulo on-chain transaction fees).
 ///
 /// Note that the deserializer is only implemented for `(`[`BlockHash`]`, `[`ChannelManager`]`)`, which
 /// tells you the last block hash which was connected. You should get the best block tip before using the manager.