docs(docs/index.md): Update "mocha.opts" documentation
docs(docs/index.md): Update "mocha.opts" documentation

Added information on file content, noting support for comment/blank lines.

outsideris/mocha,outsideris/mocha,boneskull/mocha,mochajs/mocha,adamgruber/mocha,mochajs/mocha,igwejk/mocha,mochajs/mocha,igwejk/mocha,boneskull/mocha,boneskull/mocha,adamgruber/mocha,adamgruber/mocha