[FEEDBACK] The `2-digit` datetime field option values need to be quoted

[eemeli]

While writing tests for :date, :time, and :datetime, I happened to note that the option values 2-digit are the only ones included by default which do not satisfy the unquoted-literal rule, so they need to be |2-digit| quoted.

It might be helpful to explicitly note this in the docs.

[aphillips]

I think we should fix the option to not require quotes. Yes, it'll be different from the JS option. But there doesn't seem to be a reason to install this tripping hazard in the default registry.

[aphillips]

@aphillips to add a note to the spec.