High level API docs

[nh2]

Based on #104.

This implements @simonmar's request from https://github.com/simonmar/async/pull/104#issuecomment-580152749:

The documentation should start by introducing withAsync, and only later should talk about async and wait. It's a bigger restructuring, but I think starting the docs with some code that you shouldn't use is not the right thing at all

There are 2 commits in here on top of #104 (see their commit messages):

1. Improve the prose in the introduction, add various smaller fixes.
2. Re-order the haddock sections to make the point even clearer.

[nh2]

I've updated the PR to implement the feedback.

[simonmar]

Thanks!