docs(ui-drilldown): clarify usage of `as` prop for Drilldown.Option

[balzss]

Closes: INSTUI-3926

test plan:

• open docs for Drilldown component
• check props for Drilldown.Option -> it should be clear how to use the as prop
• check if you can set the as prop following the docs and inspect the element if it had worked (e.g. try setting it as="form")

[github-actions[bot]]

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