docs: note that if globals are not injected afterEach will not run (#645)

* merge master * docs: note that if globals are not injected afterEach will not run Resolves https://github.com/testing-library/react-testing-library/issues/796 * remove unnecessary code * remove yarn.lock

Author: MatanBobi

docs/react-testing-library/api.md

@@ -259,8 +259,9 @@ expect(firstRender).toMatchDiffSnapshot(asFragment())
 Unmounts React trees that were mounted with [render](#render).
 
 > Please note that this is done automatically if the testing framework you're
-> using supports the `afterEach` global (like mocha, Jest, and Jasmine). If not,
-> you will need to do manual cleanups after each test.
+> using supports the `afterEach` global and it is injected to your testing
+> environment (like mocha, Jest, and Jasmine). If not, you will need to do
+> manual cleanups after each test.
 
 For example, if you're using the [ava](https://github.com/avajs/ava) testing
 framework, then you would need to use the `test.afterEach` hook like so: