Update Date Time node description to better illustrate how to use it

[QilongTang]

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Purpose

Per https://jira.autodesk.com/browse/DYN-3434, Update Date Time node description to better illustrate how to use it. There are lots of Date Time formats, so instead of trying them all, we should provide better documentation of how to stick to the default one. Let me know what you think.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

Update Date Time node description to better illustrate how to use it

Reviewers

@DynamoDS/dynamo

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