docs: describe yargs.async API surface

[bcoe]

Background

@mleguen and @petrgrishin, in #1473, #1488, #1420, and #1487, have kicked off a conversation about what a better thought out asynchronous API surface for yargs would look like ... After dragging my feet initially, I now officially find myself roped in 😆 ...

and getting a little bit excited about the potential of this cleaning up some of the ugly nooks and crannies of the yargs API.

What is This?

As suggested in #1487, I think we should start by collaborating on a document that thoroughly describes yargs' async API surface, before we dive in with the implementation

[bcoe]

@petrgrishin I have slight concerns about the mixing of callbacks and promises in #1473, curious what your thoughts are on this PR as a potential direction for async behavior, mainly getting a promise back from a command if the command has a promise.

[bcoe]

@mleguen, looks like we're opting to start with @petrgrishin's implementation? I still think this might be worth revisiting, would potentially be nice to have a way to opt into promises from yargs rather than using callbacks, but let's see if folks like the approach released in yargs@15.1.0?