Restore embedded documentation in TS and Flow definitions

[novemberborn]

I completely refactored the type definitions in #1670 without retaining the embedded documentation. Sorry about that 😞

It'd be great to restore and even improve the embedded documentation. The index.d.ts and index.js.flow files are quite similar to each other, so work should start on one and then by copied to the other. Screen grabs of how the documentation is displayed in code editors would be useful so we can review how much documentation should be included.

[kugtong33]

@novemberborn , a lot has happened in the past days when I was gone, I think it's my time to dive into documentation, do you need help with this one?

[novemberborn]

@kugtong33 that'd be great, thanks!

[kugtong33]

@novemberborn

How does ifError differs from falsy if both checks the value then fails it when value is truthy?

[kugtong33]

I hope this gif is helpful, I find it fast myself, lol

Using ava master branch as dependency version and vscode 1.20.1

...
"devDependencies": {
    "ava": "ssh://git@github.com:avajs/ava.git#master"
}
...

[novemberborn]

How does ifError differs from falsy if both checks the value then fails it when value is truthy?

I know… I've just opened #1720.