Update docs to say that time can be missing from isodatetime/datetime dtype?

[rly]

Useful for NWB: NeurodataWithoutBorders/nwb-schema#542

Current docs: https://hdmf-schema-language.readthedocs.io/en/latest/description.html#dtype

Proposal:

• Allow date in the current isodatetime / datetime definition. Example: date of birth (time is rarely relevant and rarely tracked)

Alternative:

• Define a new date dtype (linkml analog: https://linkml.io/linkml-model/latest/docs/Date/ )
• Define a new dateordatetime dtype (linkml analog: https://linkml.io/linkml-model/latest/docs/DateOrDatetime/ )