Change and document payment method and means in MX

[cavalle]

• Documents how payment method (MethodPago) is inferred from the presence or absence of advances
• Expects the payment means keys to be given in the advances (instead of in the instructions) when an invoice is prepaid to map to a FormaPago value.
• Doesn't expect a payment means key in non-prepaid invoices. The FormaPago in that case must always be set 99 (Por definir)
• Allows the payment means key to be given in the instructions temporarily until the users transition to use the advances

[cavalle]

Thanks a lot @samlown! Copy suggestions are always very much appreciated!