docs: Clarify that http_proxy is not used for HTTPS requests

[aaronk6]

Update documentation: Clarify that http_proxy is not used for HTTPS requests

While experimenting with proxy settings in a Butane config, I noticed that https_proxy must be explicitly set for HTTPS requests to go through the proxy. Setting only http_proxy will proxy plain HTTP requests, but HTTPS requests still go out directly.

However, the documentation previously stated that http_proxy would also be used for HTTPS requests unless overridden by https_proxy. This is incorrect, as confirmed on Matrix.

This PR updates the documentation to reflect the actual behavior.

[aaronk6]

Thanks. The Flatcar docs copy of the Ignition config spec also needs to be fixed.

Oh, missed that. Just updated my PR.

Also, we should do the same change upstream in the Ignition specs and Butane specs (linked specific files here but there's a copy for each variant/version)

I’ll look into this, too.

[pothos]

Thanks, looks good, I wonder if we should wait until this is also merged upstream (Butane and Ignition repos) or it might get overwritten once we need to sync a newer version of the specs?