extra_args not documented

[metaperl]

extra_args apparently has something to do with "extra command-line arguments that don't set config values" but nowhere is this functionality documented. I noticed it in a towards data science article.

tutorial and reference documentation would be ideal.

another issue does reference extra_args also.

[metaperl]

Also, what motivated extra_args? Why would you want extra command-line arguments that don't set config values?

[azjps]

Docs have been updated a bit, feel free to submit a PR to improve docs if you encounter anything missing.