Unreachable code defaults

Apparently, since 2.9.1, unreachable code is allowed by default, you have to explicitly set allowUnreachableCode to false in tsconfig.json to get the old behavior.

This doesn't match what's described in the docs.

Quick repro :
package.json

{
  "name": "tmp",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "typescript": "2.9.1"
  }
}

tsconfig.json (default from tsc --init)

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "strict": true,
    "esModuleInterop": true
  }
}

index.ts

export const a = (): boolean => {
    return true;
    return false;
};

yarn tsc outputs no warning as of 2.9.1

Start with the compiler-options documentation linked in the issue and reproduce the behavior using the shown tsconfig.json and index.ts under TypeScript 2.9.1. Compare the documented default for allowUnreachableCode with the compiler's actual behavior and determine whether the documentation or compiler should change. Done means the default behavior and documentation agree, with the reproduction covered by an appropriate test.