Add documentation for reactivity functions

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What problem does this feature solve?

When building anything that relies on the new awesome reactivity API, one is constantly calling the different functions like ref, reactive, computed etc. (obviously). These function are explained on https://v3.vuejs.org/api/reactivity-api.html, true.

But some of them are lacking actual code documentation - which means the generated type definitions are lacking documentation, as well.

It would be a nice touch if we had those docs, though, as it would allow IDEs to provide us with quick documentation lookup!

What does the proposed API look like?

Given some explanations on the inner workings, I could provide some proper code documentation. It's not a big thing, but it might make life easier for quite a few people out there :)

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As an example how this could (should?) look, here's me hitting F1 in intelliJ while the cursor is on the computed method call:

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PR will come tomorrow as it's too late now 😅

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Thanks to @atomjar for encouraging me to finally stop procrastinating and start contributing instead 🦄