f msg document types

Author: TheBlueMatt

lightning/src/blinded_path/message.rs

@@ -404,6 +404,10 @@ pub enum OffersContext {
 		/// needs to be "stored" by a payment recipient for their own internal use, provided back to
 		/// them with the payment.
 		///
+		/// Payment metadata is stored as a map from a numeric key to an arbitrary byte array value.
+		/// This allows for several types of metadata to be stored attached to a single payment. In the
+		/// future some optional features of LDK may use some keys.
+		///
 		/// Note that because this is included in the payment onion, its size must be tightly
 		/// constrained. More than a few hundred bytes and the payment will be entirely unpayable (with
 		/// limited routing options as size increases). Further, any data placed here will increase