[ANCHOR-749] Add SEP-38 validation documentation

openapi/anchor-platform/main-callbacks.yaml

@@ -208,6 +208,26 @@ paths:
         Either `sell_amount` or `buy_amount` will be included in requests as parameters, but never both. In the same way,
         either `sell_delivery_method` and `buy_delivery_method` may be included in requests, but never both, since either
         `sell_asset` or `buy_asset` is a Stellar asset.
+
+
+        Upon receiving the response, the Anchor Platform will validate the amount and price of the response.
+
+        If the validation fails, the Platform will respond to the client application's request with a HTTP status
+        code of `502 Bad Gateway`.
+
+        The `sell_mount`, `buy_amount`, `price`, and `fee` are validated as follows:
+
+        - if `rate.fee` exists,
+          - `rate.fee.asset` must be defined with proper `significant_decimals` in the asset configuration.
+          - `rate.fee.total` must equal to the sum of `rate.fee.details.amount`.
+          - if the `rate.fee.asset == rate.sell_asset`, `sell_amount ~= price * buy_amount + fee` must hold true.
+          - if the `rate.fee.asset == rate.buy_asset`,  `sell_amount ~= price * (buy_amount + fee)` must hold true.
+        - if `rate.fee` does not exist, `sell_amount ~= price * buy_amount` must hold true.
+
+        The `~=` is defined as equality within rounding error.
+            The rounding error is defined as `10^(-significant_decimals)`
+
+
       tags:
         - Rates
       x-seps: