MEDIUM: config: warn when expose-experimental-directives is used for no reason

MEDIUM: config: warn when expose-experimental-directives is used for no reason

If users start to enable expose-experimental-directives for the purpose
of testing one specific feature, there are chances that the option remains
forever and hides the experimental status of other options.

Let's emit a warning if the option appears and is not used. This will
remind users that they can now drop it, and help keep configs safe for
future upgrades.