Add documentation about allowed target assets

docs/flows/crypto-onramp.mdx

@@ -27,6 +27,7 @@ The crypto on-ramp flow allows a user to add funds from a credit or debit card t
   - `paymentMethod` (_optional_): An object configuring the pre-selected payment method.
     - `network`: Payment method network (e.g.: `apple-pay`, `google-pay`, `pix` or `card`).
 - `target` (_optional_): An object configuring recipient wallet.
+  - `allowedAssets` (_optional_): Set of cryptoassets that the user can set as target asset.
   - `amount` (_optional_): Amount the user will receive.
   - `asset`: Cryptoasset the user will receive.
   - `network`: Network of the receiving asset.
@@ -44,6 +45,12 @@ At least one of `source.amount` or `target.amount` _must_ be provided, but not b
 The supported values for `source.asset`, `target.asset`, `target.network`, `target.tag` and `target.priority` can be found using our [REST API](../rest-api.md), via the [assets endpoint](https://api.topperpay.com/assets/crypto-onramp).
 :::
 
+:::caution
+If `target.allowedAssets` is set, and you define `target.asset` with an asset that is not allowed, the bootstrap will fail.
+
+If `target.allowedAssets` is set and `target.asset` is not, the `target.asset` will default to the first asset on `target.allowedAssets`.
+:::
+
 :::caution
 In the `sandbox` environment, Topper has limited funds on testnets. If you make an order using an asset we have no funds for, the order will be stuck indefinitely trying to create the blockchain transaction.
 We recommend that you use `XRP` for testing purposes when integrating Topper since we have a larger quantity of them. If you prefer to test with another asset and network, please reach out to us beforehand so that we can guarantee funds on our pool and give you a smoother integration experience.
@@ -73,6 +80,7 @@ We recommend that you use `XRP` for testing purposes when integrating Topper sin
     }
   },
   "target": {
+    "allowedAssets": ["BTC", "ETH", "XRP"]
     "address": "0xb794f5ea0ba39494ce839613fffba74279579268",
     "asset": "ETH",
     "network": "ethereum",