[HttpClient] Document new option auto_upgrade_http_version

[ajgarlag]

Fix #21498

[nicolas-grekas]

Suggested change

	See :ref:`auto_upgrade_http_version <auto-upgrade-http-version>` option for details about how the HTTP protocol version upgrade works.
	See :ref:`auto_upgrade_http_version <auto-upgrade-http-version>` option for details about how the HTTP protocol version selection works.

[nicolas-grekas]

Suggested change

	possible.
	desired.

[nicolas-grekas]

I'd suggest removing this part, I'm not sure it's worth reading/learning

Suggested change

	The automatic HTTP version upgrade is always disabled for HTTP/1.0 requests,
	regardless of the option value.

[ajgarlag]

I'd prefer to have this documented.

[javiereguiluz]

I agree with @ajgarlag here. I changed this to a note directive. I know HTTP/1.0 is irrelevant these days, but this is an edge-case that can turn into user confusion and frustration if we don't mention it explicitly. Thanks.

[javiereguiluz]

Thanks Antonio! Your contribution was very nice, but we made some rewords/reformatting while merging.

Specifically, we don't add contents in versionadded directives because they are automatically deleted in upper branches. So, we moved all the content outside and added an example, while keeping the versionadded directive only to mention that this option was introduced in 7.4. Thanks!