Clarify docs for TS tests & test folder locations

Created by: justingrant

This PR makes the following improvements to existing documentation about location and file suffxes of test files:

• Clarifies that CRA now (after CRA 2.1, AFAIK) supports tests that are written in TypeScript. This was important to document because Google misdirects users to pages like https://dev.to/thetrevorharmon/configuring-jest-and-enzyme-in-create-react-app-on-typescript-17mj or https://github.com/basarat/typescript-book/blob/master/docs/testing/jest.md#step-2-configure-jest which propose solutions using custom Jest configs that aren't needed (and sometimes don't work!) within a modern CRA environment.
• Clarifies that a __tests__ folder cannot be a peer to src. This was already implied in the docs but IMHO wasn't stated explicitly enough to avoid errors (e.g. https://stackoverflow.com/questions/49602402/create-react-app-cannot-find-tests), so I added text that's very explicit to help users avoid this failure case.
• Fixed a minor typo in existing text.