fs.readFile with encoding as 2nd argument is not documented

It is very common to call fs.readFile with the encoding name directly as the 2nd argument. Like

```
fs.readFile('my_file.txt', 'utf8', cb)
```

Yet this call signature is not clearly documented:

https://iojs.org/api/fs.html#fs_fs_readfile_filename_options_callback (link date 2015-05-26)
