I think for the /usr/local/etc/php directory, the better wording is to have it exposed as a volume so you can do a bind mount and specify overrides. Nothing in that directory is modified at run time so no need for you to define it on an external volume and make it persistent.
You can open a GitHub issue here -> https://github.com/docker-library/wordpress/issues
Tell them you want the Docker Official Wordpress Docker Image to expose the /usr/local/etc/php directory as a volume mount point so you (and other users) can bind mount your own customization files at container run time.