fixup: Improve sign_message documentation

Author: martinsaposnic

lightning/src/sign/mod.rs

@@ -932,6 +932,11 @@ pub trait NodeSigner {
 	fn sign_gossip_message(&self, msg: UnsignedGossipMessage) -> Result<Signature, ()>;
 
 	/// Sign an arbitrary message with the node's secret key.
+	///
+	/// Creates a digital signature of a message given the node's secret.
+	/// A receiver knowing the node's id and the message can be sure that the signature was generated by the caller.
+	/// An `Err` can be returned to signal that the signer is unavailable / cannot produce a valid
+	/// signature.
 	fn sign_message(&self, msg: &[u8]) -> Result<String, ()>;
 }