Add docs for waitForElementToBeRemoved

[kentcdodds]

I just noticed it's missing from https://testing-library.com/docs/dom-testing-library/api-async and https://testing-library.com/docs/guide-disappearance 😬

I should've added those docs but I forgot. Anyone want to add them? I'm currently low-bandwidth for the next while.

[kentcdodds]

Here's the source code: https://github.com/testing-library/dom-testing-library/blob/master/src/wait-for-element-to-be-removed.js

The tests: https://github.com/testing-library/dom-testing-library/blob/master/src/__tests__/wait-for-element-to-be-removed.js

More tests: https://github.com/testing-library/dom-testing-library/blob/master/src/__tests__/wait-for-element-to-be-removed.fake-timers.js

And the TS defs: https://github.com/testing-library/dom-testing-library/blob/master/typings/wait-for-element-to-be-removed.d.ts

[Pklong]

I'd be happy to do this.

[bcarroll22]

@alexkrolick we can close this, yeah?