[PR] Document async function return type
[PR] Document async function return type

Summary:
Adds a note about how to type the return value of an async function.

Fixes #6001
Pull Request resolved: https://github.com/facebook/flow/pull/6000

Reviewed By: nmote

Differential Revision: D13324254

Pulled By: mvitousek

fbshipit-source-id: 6dee5212702c147529c367b6d6ac041931ee3507

mroch/flow,popham/flow,mroch/flow,mroch/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,popham/flow,mroch/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,popham/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,gabelevi/flow,popham/flow,popham/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,popham/flow,gabelevi/flow,mroch/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,popham/flow