Clean up SpendableOutputDescriptor::StaticOutput docs somewhat

As suggested by devrandom.

Author: TheBlueMatt

lightning/src/chain/keysinterface.rs

@@ -49,7 +49,8 @@ use ln::msgs::DecodeError;
 pub enum SpendableOutputDescriptor {
 	/// An output to a script which was provided via KeysInterface directly, either from
 	/// `get_destination_script()` or `get_shutdown_pubkey()`, thus you should already know how to
-	/// spend it. No secret keys are provided as rust-lightning was never given any key.
+	/// spend it. No secret keys are provided as the generating ChannelMonitor was never given any
+	/// secret key material.
 	/// These may include outputs from a transaction punishing our counterparty or claiming an HTLC
 	/// on-chain using the payment preimage or after it has timed out.
 	StaticOutput {