Define promise-like/callbacks in the docs

[asklar]

We should:

• Define what a callback is. Is it strictly a void (*) ()? a std::function? is it anything with an operator() ?
• What types they take/return
• What the difference between 1 or 2 is
• When you would use a callback, two callbacks or a promise

Originally posted by Alexander Sklar (@asklar) in #280 (comment)

[chrisglein]

The default path is more complicated than it may need to be (powerful, but confusing). We may have plans to streamline this, but in the meantime we need to document what we have.