[ObservableObject] Refactor ObservableObject docs a bit?

[citelao]

Today, the Observable Object docs make no mention of INotifyPropertyChangedAttribute or ObservableProperty. I began this draft to remedy this gap, only to realize that the API documentation is also fairly out of date.

Happy to help revise.

[learn-build-service-prod]

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[learn-build-service-prod]

Learn Build status updates of commit e4e8638:

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