[clipboard-apis] Be explicit about before* events (#24)

> The before* events can be used to override the internal logic.

It would increase clarity if all event names were to be written in full here.

> The beforecopy, beforecut, and beforepaste events can be used to override the internal logic.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/clipboard-apis/issues/24