Document the expiresIn-to-expiry_ledger formula formally in the API reference

[phertyameen]

Location

docs/api-reference.md

Problem

The conversion formula currently only lives in the README's temporary-workarounds section; once implemented properly (section A), it deserves permanent, prominent documentation in the actual API reference integrators would consult.

Acceptance Criteria

• Formula and its assumptions (ledger close time) documented alongside the CreateAccountDto.expiresIn field
• Cross-linked from the README once the temporary-workarounds section is eventually removed
• Kept in sync if the underlying implementation issue changes the approach