Provide a simpler exlanation and examples about when promises "start".
Make it clear that the promise's body executes after the promise is created,
without the need to call `then` or `done` on it. Also clarify that async
tasks that a being kicked off from a promise's body behave the same way
regardless of whether PromiseKit is being used or not.
This because `Promise(value:)` is selected for PMK-4’s pending initializer due to trailing closure syntax and the fact that `T` could be anything. I guess this isn’t a Swift bug, but geez.
This is a real problem for people migrating to 5 from 4, such that I have decided 5 must be deprecated.
The severity is due to Swift giving a very misleading error diagnostic when it uses `Promise(value:)`, because as you can guess you end up with:
Promise<(T->Void, Error->Void)>
As your inferred type.
Because we no longer have ambiguity between `Promise { seal in` and `Promise(value:` I have removed the `.pending` parameter for that usage which will also aid migration.
In some cases this leads to less nice code, but mostly it’s fine due to being able to do `.value(1)` where `Promise` can be inferred. The trade off is worth it though as PromiseKit should be delightful to use and as it stands, it is a pain due to how Swift is.
