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

从链接的 Test Utils 文档中的 renderIntoDocument() 开始，并查看相关的入口点 unmountComponentAtNode()。记录分离的 DOM 树和组件实例如何被清理、容器来自哪里、如何处理其他树，以及测试如何验证清理工作。