Document the odd handling of Arabic locales and their digits (ar_DZ vs. ar_EG vs. ar)

[AbdulRahmanAlHamali]

At this line: https://github.com/dart-lang/intl/blob/4b385517037485e3243d6915453215af315add27/lib/number_symbols_data.dart#L64

You should use the Eastern Arabic zero numeral: https://en.wikipedia.org/wiki/Eastern_Arabic_numerals

This is implemented in dates, but not in NumberFormat