Documentation work on vDatetime

[niccokunzmann]

The vDatetime documentation has been improved by #917. Some parts still need work.
To close this issue:

• Follow the docstring pattern of icalendar.prop.html to render its format as a proper definition list. The current rendering of icalendar.prop.vDatetime is malformed.
• Fix typographical mistakes.
• Any other suggested formatting improvements.

This is a good introductory issue. Please ask @stevepiercy for anything you need to get started on contributing.
As developers of icalendar, you can help us with your questions to make the project more contributor friendly and improve our documentation!
@natashamm - because you contributed to vDatetime, would you like to work on this?