[expr.await] Define 'suspend' as a term of power for coroutines

[AlisdairM]

This PR addresses #7377.

Many parts of the standard refer to whether a coroutine is suspended as if it were a term of power, but there is no \defn that defines it as such. This PR places the definition on the only use that I could find where a suspended state was not defined in terms of other suspended states, so may be the transition into suspension. It is likely that folks with a better eye to Core wording will know a better place, that may need a little rewording to sneak in the word 'suspended' to properly define it.