Mark SpendableOutputDescriptor::to_psbt_input as no-export

Its honestly likely not all that useful as its not materially
interoperable with other PSBT libraries. Instead, users should
simply fetch the full PSBT and use the inputs from it as they see
fit.

Author: TheBlueMatt

lightning/src/sign/mod.rs

@@ -275,6 +275,9 @@ impl SpendableOutputDescriptor {
 	///
 	/// Note that this does not include any signatures, just the information required to
 	/// construct the transaction and sign it.
+	///
+	/// This is not exported to bindings users as there is no standard serialization for an input.
+	/// See [`Self::create_spendable_outputs_psbt`] instead.
 	pub fn to_psbt_input(&self) -> bitcoin::psbt::Input {
 		match self {
 			SpendableOutputDescriptor::StaticOutput { output, .. } => {