docs(kyc): note that receivers can require fields at payment time

The page said the hosted flow collects whatever `POST /customers` omitted.
That holds for the KYB review, but not for payment: receivers advertise their
own mandatory sender fields, and a missing one fails `POST /quotes` with
MISSING_MANDATORY_USER_INFO and no prompt anywhere. `businessType` is the case
that bites — GBP receivers require it, so an approved business can still be
unable to pay into them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: JasonCWang

mintlify/snippets/kyc/verification-options.mdx

@@ -77,6 +77,14 @@ Everything else is optional to the schema, but three groups matter in practice:
 - **Currency-driven fields** — `GET /config` returns `supportedCurrencies`, each with a `providerRequiredCustomerFields` list. Anything listed for a currency the business will use must be supplied.
 - **Everything the review needs** — `address`, `registrationNumber`, `entityType`, `countriesOfOperation`, `businessType`, `purposeOfAccount`, `sourceOfFunds`, and the expected-activity fields aren't enforced at creation. Whatever you omit, the applicant is asked for in the hosted flow.
 
+<Warning>
+The hosted flow only backfills what the *review* needs. Some of these fields are also read at **payment** time, and there the omission surfaces as a failed quote rather than a prompt.
+
+Each receiver declares which sender fields it requires, so the requirement depends on where the business is paying rather than on where it was onboarded. When a receiver requires a field the business record doesn't have, `POST /quotes` fails with `MISSING_MANDATORY_USER_INFO` before the payment is attempted. `businessType` is the one to watch: GBP receivers require it, and an approved business with no `businessType` still can't pay into them.
+
+Populate it at creation, or fix it after the fact with `PATCH /customers/{id}`. Passing `senderCustomerInfo` on the quote also satisfies the check, but only for that one quote — it doesn't update the customer.
+</Warning>
+
 Individual customers need `customerType: INDIVIDUAL`; `fullName` must contain both a given and a family name.
 
 ## Status transitions