EventLoopPromise didn't point out it has reference semantics (#612)
EventLoopPromise didn't point out it has reference semantics (#612)

Motivation:

Sometimes people assume that all `struct`s have value semantics but
that's not true. We should still point that out.

Modifications:

point out that EventLoopPromise has reference semantics

Result:

clearer docs
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio