Improve Docs

[elibroftw]

The docs don't indicate how the serde serialization will be performed on ObjectID or Datetime. I suggest improving the documentation for objectID and Datetime to show how access the fields in a frontend that expects json.

I also ask to improve the datetime documentation to include how to show the date in different formats.

[patrickfreed]

Thanks for filing this issue, I've gone ahead and updated the docs for DateTime and ObjectId to be clearer about how they interact with serde. Let us know if you have any further feedback.