Add more required types or explanation of limits to not adding fields

[robindunkerley]

Issues

1. it is not clear to developers that the lineItem field must be included on the invoice object to avoid validationException errors.

2. It is not clear that the accountCode field must be included on the lineItem object in order to create invoices with a status of Invoice.StatusEnum.AUTHORISED without experiencing validationException errors.

3. In order to send invoices using the emailInvoice method, invoices must have a status of Invoice.StatusEnum.AUTHORISED. This is not clear in and of itself, however, a solution to issue 2 would reduce unclear errors for developers.

Suggested solutions

1. Make lineItem a required field on type Invoice.

2. Make accountCode a required field on type lineItem OR include an explanatory comment in lineItem.d.ts highlighting its relation to adding a status of authorised.