[editing] Reference actions in clipboard API spec for execCommand copy/cut/paste (#61)

See discussion in https://github.com/w3c/clipboard-apis/issues/16 - in particular https://github.com/w3c/clipboard-apis/issues/16#issuecomment-125605131 and consider replacing the current text about copy/cut/paste in your execCommand spec with one saying you should invoke the actions as defined by my spec.

Thanks :)

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