Update docs for Bolt12SemanticError::InvalidAmount

Added doc update to clarify that `InvalidAmount` now also covers
cases where the amount in an invoice does not match the expected
amount specified in the associated `InvoiceRequest` or `Refund`.

Author: slanesuke

lightning/src/offers/parse.rs

@@ -144,6 +144,9 @@ pub enum Bolt12SemanticError {
 	/// An amount was expected but was missing.
 	MissingAmount,
 	/// The amount exceeded the total bitcoin supply.
+	///
+	/// This error can also occur if the amount in an invoice does not match the expected
+	/// amount specified in the associated `InvoiceRequest` or `Refund`.
 	InvalidAmount,
 	/// An amount was provided but was not sufficient in value.
 	InsufficientAmount,