docs: remove "computed" from composable usage restrictions

[cernymatej]

This PR removes the mention of linking computed properties to the instance of a component in order to be stopped. If I'm not mistaken, this is no longer how computed properties work after the reactivity refactor introduced in 3.5 https://github.com/vuejs/core/pull/10397. Computed properties now do not need to be stopped and can be garbage collected once they are no longer in scope.

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