Clarify what special keys are supported by `userEvent.keyboard` API

[hi-ogawa]

Describe the bug

related: #6769, #5770 (comment)

I'm not entirely sure what works now. We need to understand the difference between providers and clarify in the documentation if there's a provider side limitation. Also add tests to ensure the known behaviors.

Reproduction

na

System Info

na

Used Package Manager

npm

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