Improve/create documentation for ReactTestUtils.renderIntoDocument()

This issue was originally reported by @the-spyke via facebook/react/issues/10330

There is almost none documentation about how to actually use .renderIntoDocument() method. I can't find find answers on those questions on Jest's website or enzyme's (I know that it's not Facebook) either.

• If I render a component into a DOM in a test, how will it be destroyed?
• Should I clean up by myself?
• What tools to use for this?
• Just .unmountComponentAtNode()?
• Where I will get the container to pass in?
• What about additional component trees like in Popovers?
• What are the best ways to test that there aren't any parts left after my component were unmounted?

It will be nice to see such information before people will met memory leaks in their tests, and also help to establish good practices for writing tests.

Additional notes:

• Calling .renderIntoDocument() returns you an instance
• This instance was rendered into freshly created DOM element, which now represents a detached tree
• There's also a reference to the instance in ReactDOM's internal map
• So, even if you remove all your references to this instance, on every call to .renderIntoDocument() you will get 1 detached tree hanging in memory + 1 instance of a React component with all its props not available for garbage collection

從連結的 Test Utils 文件中的 renderIntoDocument() 開始，並檢視相關的進入點 unmountComponentAtNode()。記錄分離的 DOM 樹和元件實例如何被清理、容器來自何處、如何處理其他樹，以及測試如何驗證清理工作。