In reference, form fields, 'money' type, possible currency options are not explained

[thepurpleblob]

For the 'money' field type, one of the options is 'currency'. The documentations says that the default is 'EUR' but does not say what the other possible options are. I'm guessing that it will be the ISO currency codes but, for those who aren't aware of that and for reassurance, it would be useful to be explicit I think.

[wouterj]

The MoneyType uses the \NumberFormatter class which is included in the intl library (source). This class requires a "3-letter ISO 4217 currency code" as the currency (source).

I think it is great to add this in the docs, I will create a PR today.

[weaverryan]

PR merged in - thanks everyone!