docs(api): point out that methods required by outputFileSystem (#1265)

The `mkdirp` and `join` methods are also needed by `outputFileSystem`.

Author: mc-zone

content/api/node.md

@@ -277,4 +277,4 @@ compiler.run((err, stats) => {
 });
 ```
 
-T> The output file system you provide needs to be compatible with Node’s own [`fs`](https://nodejs.org/api/fs.html) module interface.
+T> The output file system you provide needs to be compatible with Node’s own [`fs`](https://nodejs.org/api/fs.html) module interface, which requires the `mkdirp` and `join` helper methods.