Update documentation on transaction speed

[ndeet]

As a note for anybody that wants to tackle some low hanging fruit I just stumbled across this for the VirtueMart documentation. We do not really describe how the "transaction-speed" settings work in the store configuration. What we have here https://docs.btcpayserver.org/FAQ/Stores/#consider-the-invoice-confirmed-when-the-payment-transaction seems also to be outdated as we now have processing (old: paid) and settled (old: confirmed) as invoice states.

[pavlenex]

If someone wants to tackle this, where can they find information on what transaction speed is and how it works?

I only found discussion between @NicolasDorier and you here, but I don't think it's still clear enough.

[ndeet]

The discussion was more about the greenfield API which is different than the store settings and some legacy from BitPay API. It is clearer now what speedPolicy on Greenfield mean, so a can do that.

I will tackle this as well.