I suppose this question is meant for the devs, but just in case: I suggest to just show (display) it. Because right below there's Open Configuration File and the instructions (how to override) are above that button which makes it easy to both see the current setting and know how to change it without reading documentation. The only thing that's missing (see other parts of the GUI) would be advice that a restart is required for the new setting to to take effect. (In case dbcache isn't set in bitcoin.conf, I assume the default value would be displayed).