Document undocumented required PHP extensions

There are a few required PHP extensions that are not mentioned in the system requirements documentation: https://docs.civicrm.org/installation/en/latest/general/requirements/#php-extensions.

It might be that these come as standard in most PHP distributions but it is probably worth documenting them.

This Dockerfile shows what we needed to install to get CiviCRM working using the official Docker PHP image: https://lab.civicrm.org/michaelmcandrew/civioneclick-docker/-/blob/main/dependencies/Dockerfile. Look for the docker-php-ext-install lines.