Add documentation for $data

[todeveni]

No description provided.

[schlessera]

Thanks for the pull-request, @todeveni.

However, this PR makes a change in a file that is auto-generated. The documentation you can find here is automatically extracted out of the source files of the respective packages.

For the above change, you need to make a PR against the wp-cli/wp-cli package: https://github.com/wp-cli/wp-cli/blob/v2.4.0/php/utils.php#L626-L650 instead.