[Accepted] SE-0390: Noncopyable structs and enums

@Joe_Groff @Michael_Gottesman @kavon I've posted an update to SE-0390: Noncopyable structs and enums; Deinitialization #2432. The proposal was accepted, but a few people noticed incorrect wording in the definition of strict lifetimes for noncopyable types. This update makes the proposal consistent with the implementation and with what was discussed in the this review thread. In the process, I clarified the related example. I renamed the example's noncopyable struct to File because current and future FileDescriptor types will likely be copyable.