`timing` deprecated on Create Transfer; docs corrected

Broker API

Deprecates the timing field on the Create Transfer request. The field has no effect and will be removed on 2026-08-26; stop sending timing in requests. There is no replacement.

Also corrects the Create Transfer documentation to match actual behavior:

• Documents the supported transfer matrix: ach supports both INCOMING (deposit) and OUTGOING (withdrawal); wire supports OUTGOING only.
• Notes that ach transfers require a relationship_id, and wire transfers require a bank_id whose bank relationship is in APPROVED status.
• Clarifies sandbox behavior: ACH deposits and withdrawals settle instantly, while outgoing wire withdrawals are simulated asynchronously and complete on weekdays only.
• Adds 400, 403, and 422 response definitions.
• timing is no longer listed as required.

Affected Endpoints

• POST /v1/accounts/{account_id}/transfers
• GET /v1/accounts/{account_id}/transfers