docs: describe rule-based accounts in sandbox

openapi/paths/internal_accounts.yaml

@@ -18,6 +18,12 @@ post:
     in `USD` for individual customers, and must be enabled for your platform
     before this endpoint accepts requests.
 
+    Sandbox platforms can create rule-based accounts and exercise the whole
+    flow. The account number is generated locally rather than issued by a
+    partner bank, and `POST /sandbox/internal-accounts/{accountId}/fund` stands
+    in for a settled deposit — funding the account triggers its sweep just as a
+    real deposit would.
+
     Creating an account mints a new account number that cannot be reversed, so
     an `Idempotency-Key` header is required. A retry carrying the same key
     returns the account created by the first request with a `200` instead of a

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

@@ -8,6 +8,21 @@ 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 forwarded to the rule's
+    destination and the account is left at zero. A balance below the corridor
+    minimum is returned to the payer instead, and `SWEEP.FAILED` is delivered
+    with `reason: BELOW_MINIMUM` — fund a rule-based account with a small
+    amount to rehearse that path.
+
+    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