Document data_file configuration

[Yoshiyuki Kinjo]

data_file is not documented in https://scoutapm.com/docs/ruby/configuration but still working fine.

Setting data_file is very important for us since file is created inside Rails app directory when server shuts down and elastic beanstalk platform does not like such behavior.
https://repost.aws/questions/QUiqL7h2trQwuV7KtjtBDc0Q/elastic-beanstalk-deployments-periodically-failing-var-app-current-directory-not-empty-amazon-linux-2-3-4-1

Please document data_file option so that someone like us can avoid such problem.