📝 Docs: Document that mocha will search for mocha configs by going through parent directories

Mocha currently searches through parent directories to find a mocharc config: https://github.com/mochajs/mocha/blob/37deed262d4bc0788d32c66636495d10038ad398/lib/cli/options.js#L182

But this behavior isn't documented in https://mochajs.org/#configuring-mocha-nodejs, which only says to create the mocharc in your package's root directory.