add __doc__ string documentation for vDatetime

[natashamm]

Description

Issue #742 for vDatetime. I also moved some of the docstrings/examples for the from_ical method for vDatetime to match the docstrings for the other properties.

Checklist

• I've added a change log entry to CHANGES.rst.
• I've added or updated tests if applicable.
• I've run and ensured all tests pass locally by following Running Tests.
• I've added or edited documentation, both as docstrings to be rendered in the API documentation and narrative documentation, as necessary.
• I've added myself to docs/credits.rst as a contributor in this pull request or have done so previously.

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📚 Documentation preview 📚: https://icalendar--917.org.readthedocs.build/

[coveralls]

Pull Request Test Coverage Report for Build 18580240887

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.838%

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Totals
Change from base Build 18555550103:	0.0%
Covered Lines:	7940
Relevant Lines:	8107

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💛 - Coveralls

[stevepiercy]

This is the more familiar term.

Suggested change

	- Add ``__doc__`` string documentation for ``vDatetime``. See `Issue 742 <https://github.com/collective/icalendar/issues/742>`_.
	- Add docstring for ``vDatetime``. See `Issue 742 <https://github.com/collective/icalendar/issues/742>`_.

[stevepiercy]

Please take a look at how this renders at https://icalendar--917.org.readthedocs.build/en/917/api.html#icalendar.prop.vDatetime. You'll see it doesn't follow the patterns of existing property docstrings, such as https://icalendar--917.org.readthedocs.build/en/917/api.html#icalendar.prop.vDate. Please build the docs locally to review and test before pushing commits to correct the display issues. Thank you!