Drop doc comments on ChainMonitor trait impl methods

In general, doc comments on trait impl blocks are not very visible
in rustdoc output, and unless they provide useful information they
should be elided.

Here we drop useless doc comments on `ChainMonitor`'s `Watch` impl
methods.

Author: TheBlueMatt

lightning/src/chain/chainmonitor.rs

@@ -654,12 +654,6 @@ where C::Target: chain::Filter,
 	    L::Target: Logger,
 	    P::Target: Persist<ChannelSigner>,
 {
-	/// Adds the monitor that watches the channel referred to by the given outpoint.
-	///
-	/// Calls back to [`chain::Filter`] with the funding transaction and outputs to watch.
-	///
-	/// Note that we persist the given `ChannelMonitor` while holding the `ChainMonitor`
-	/// monitors lock.
 	fn watch_channel(&self, funding_outpoint: OutPoint, monitor: ChannelMonitor<ChannelSigner>) -> Result<ChannelMonitorUpdateStatus, ()> {
 		let mut monitors = self.monitors.write().unwrap();
 		let entry = match monitors.entry(funding_outpoint) {
@@ -693,8 +687,6 @@ where C::Target: chain::Filter,
 		Ok(persist_res)
 	}
 
-	/// Note that we persist the given `ChannelMonitor` update while holding the
-	/// `ChainMonitor` monitors lock.
 	fn update_channel(&self, funding_txo: OutPoint, update: &ChannelMonitorUpdate) -> ChannelMonitorUpdateStatus {
 		// Update the monitor that watches the channel referred to by the given outpoint.
 		let monitors = self.monitors.read().unwrap();