Document webpack change: disable node stuff in node targets

[webpack-bot]

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#9172

---

What kind of change does this PR introduce?
defaults change

Did you add tests for your changes?
no

Does this PR introduce a breaking change?
yes

What needs to be documented once your changes are merged?
node.__filename, node.__dirname and node.global defaults to false in node.js targets

[jeffin143]

@montogeek
In this file - https://github.com/webpack/webpack.js.org/blob/master/src/content/configuration/node.md

I guess it explicitly mentioned that node.__filename, node.__dirname and node.global defaults to false in node.js targets

false: **The regular Node.js __filename behavior**. The filename of the output file when run in a Node.js environment.

false: **The regular Node.js __dirname behavior**. The dirname of the output file when run in a Node.js environment.

The only blocker I see in closing this issue is the following

These are the defaults:

__webpack.config.js__

```javascript
module.exports = {
  //...
  node: {
    global: false,
    __filename: 'mock',
    __dirname: 'mock',
  }
};

I guess we need to correct the JavaScript snippet to change mock to false, since the above line mentions These are the defaults

But if you find that is ok, then we can close this Issue,

Let me know if that isn't the case I can go ahead and open a pr :)

[jeffin143]

friendly-ping @montogeek