JSDoc generation - generate @returns when function returns a value

_From @egamma on February 22, 2017 11:14_

In the following generated JS Doc comment I would have expected that there is also a `@returns` comment.
 
```js
/**
 * 
 * @param {any} a
 * @param {any} b
 */
function foo(a, b) {
  return 12;
}
```

_Copied from original issue: Microsoft/vscode#21109_