docs: describe rule-based accounts in sandbox

openapi/paths/sandbox/sandbox_internal_accounts_{accountId}_fund.yaml

@@ -8,6 +8,23 @@ post:
 
     This endpoint is only for the sandbox environment and will fail for
     production platforms/keys.
+
+    ### Funding a rule-based account
+
+    Funding a `RULE_BASED` account triggers its sweep, exactly as a settled
+    deposit does in production: the balance is swept to the rule's
+    destination and the account is left at zero. A balance outside the corridor
+    band moves to the account holder's own account in the same currency
+    instead, and the transaction carries `sweepCanonicalAccountId` with either
+    `sweepFailureReason: BELOW_MINIMUM` or `ABOVE_MAXIMUM` — fund a rule-based
+    account with a very small amount to rehearse that path. Either way the
+    rule-based account is left at zero; it never holds a balance.
+
+    Sandbox simulates the API contract and the routing decision, not the
+    banking rails. The quote, the corridor bounds, the destination checks and
+    the webhook are all real; the settlement legs behind them are not, so
+    timings and failure modes that originate at a partner bank cannot be
+    reproduced here.
   operationId: sandboxFundInternalAccount
   tags:
     - Sandbox