In the doc for `-X` options (https://docs.python.org/3/using/cmdline.html#id5), when an option can be triggered through an equivalent environment variable, that variable is mentioned.

An exception to that is `-X dev`, which can also be triggered by the PYTHONDEVMODE variable, but that isn't mentioned in the CLI doc.

Other missing environment variables in the CLI doc are PYTHONFAULTHANDLER and PYTHONTRACEMALLOC.

I would like to work on this issue.
Can you please suggest me specifics that I should be considering before I update the doc with the suggested inclusions.

> An exception to that is `-X dev`, which can also be triggered by the PYTHONDEVMODE variable, but that isn't mentioned in the CLI doc.

I wrote a whole new page for -X dev: https://docs.python.org/dev/library/devmode.html#devmode

This page starts with:

"It can be enabled using the -X dev command line option or by setting the PYTHONDEVMODE environment variable to 1."

> I wrote a whole new page for -X dev: https://docs.python.org/dev/library/devmode.html#devmode

That's nice, but references for PYTHONFAULTHANDLER and PYTHONTRACEMALLOC are still missing. If you think the cross refs for -X dev and PYTHONDEVMODE clutters cmdline.html, I'll remove them from the PR.

Is it worth it adding https://docs.python.org/dev/library/devmode.html to 3.8 and possibly 3.7 as well?