The documentation for configuring Authenticated Origin Pulls on the origin web-server seems to have vanished, and looks like it has been accidentally replaced by developer documentation.
Previously, the docs mentioned Apache and Nginx configuration and how to install the Cloudflare client certificate. For example, SSLVerifyClient and SSLCACertificateFile were used with Apache. The support page was Authenticated Origin Pulls – Cloudflare Help Center at support.cloudflare.com.
Now, the previous link, and all other links on this topic including on the SSL/TLS dashboard, redirect to this developer page at developers.cloudflare.com: https://developers.cloudflare.com/ssl/origin-configuration/authenticated-origin-pull
The new page contains no description of web-server configuration. It seems oriented to developers using the Cloudflare API, rather than administrators configuring a web-server.