Document --require-virtualenv flag

[tusharsadhwani]

Resolves #10456

Edit: Hey! This PR is fairly incomplete right now, it was made during a live stream to help show how one might find, fix and make PRs to open source. I'll be taking a look at the sphinx code and see why readthedocs isn't picking up the flag as well.

Is there any other place that you might want to document the --require-venv flag / env var? Do let me know, I'd be happy to write it down.

[tusharsadhwani]

Okay, I get what's going on now.

require_virtualenv is marked as a "general option", as in, it can be passed to any command. But that prevents it in showing up in readthedocs pages for pip install, for example. And I don't think that's a good thing.

[tusharsadhwani]

So here's what I'd suggest:

In the Usage section, we can have a sub-heading called "Virtualenvs", and document it there.

Like so: