fix(docs): add zero standard payment warning to FDC (#494)

Author: dineshpinto

docs/fdc/attestation-types/balance-decreasing-transaction.mdx

@@ -42,6 +42,12 @@ A transaction is considered “balance decreasing” for the specified address i
 | `spentAmount`              | `int256`      | Amount spent by the source address in minimal units (can be negative). |
 | `standardPaymentReference` | `bytes32`     | Standardized payment reference of the transaction, if available.       |
 
+:::warning[Standard Payment Reference]
+
+If a transaction has no `standardPaymentReference`, it is set to default value, thus, zero value reference should be used with caution.
+
+:::
+
 ## Verification process
 
 1. The transaction identified by `transactionId` is fetched from the **source blockchain node** or a relevant indexer.

docs/fdc/attestation-types/payment.mdx

@@ -47,6 +47,12 @@ Each supported blockchain specifies how a payment transaction should be formatte
 | `oneToOne`                     | `bool`        | Indicates if the transaction involves only one source and one receiver.                                                        |
 | `status`                       | `uint8`       | [Transaction success status](#transaction-success-status).                                                                     |
 
+:::warning[Standard Payment Reference]
+
+If a transaction has no `standardPaymentReference`, it is set to default value, thus, zero value reference should be used with caution.
+
+:::
+
 ## Verification Process
 
 1. The transaction identified by `transactionId` is fetched from the relevant blockchain node or indexer.