Clarify docs on provide_channel_parameters

Its very confusing to say that LDK will call
`provide_channel_parameters` more than once - its true for a
channel, but not for a given instance. Instead, phrase the docs
with reference to a specific instance, which is much clearer.

Author: TheBlueMatt

lightning/src/chain/keysinterface.rs

@@ -369,9 +369,11 @@ pub trait BaseSign {
 		-> Result<(Signature, Signature), ()>;
 
 	/// Set the counterparty static channel data, including basepoints,
-	/// counterparty_selected/holder_selected_contest_delay and funding outpoint. Since these are
-	/// static channel data, they MUST NOT be allowed to change to different values once set, as LDK
-	/// may call this method more than once.
+	/// counterparty_selected/holder_selected_contest_delay and funding outpoint.
+	///
+	/// This data is static, and will never change for a channel once set. For a given [`BaseSign`]
+	/// instance, LDK will call this method exactly once - either immediately after construction or
+	/// when the funding information has been generated.
 	///
 	/// channel_parameters.is_populated() MUST be true.
 	fn provide_channel_parameters(&mut self, channel_parameters: &ChannelTransactionParameters);