fix(docs): clarify defaultFn

[aaqilniz]

In the case of the defaultFn property, setting generated to true or setting required to true fails the request. This PR updates the docs and mentions not to set these options in the case of defaultFn.

The confusion arises from the fact that generated represents a field automatically generated. In the case of default, we can set the required to true but not in the case of defaultFn.

Checklist

• [ ] DCO (Developer Certificate of Origin) signed in all commits
• [ ] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [ ] Code conforms with the style guide
• [ ] API Documentation in code was updated
• [ ] Documentation in /docs/site was updated
• [ ] Affected artifact templates in packages/cli were updated
• [ ] Affected example projects in examples/* were updated

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