docs: unstable_cache clarify usage

[leerob]

It can still be used now, until "use cache" is ready.

[mllustosa]

@leerob Can we safely use unstable_cache in a production environment? Is the unstable_ now referring more to the fact that the API will change and become "use cache" or that the kv (I guess) used internally is not stable for production use?

Thanks a lot - from a huge fan! :-)

[leerob]

It's unstable insofar as the API will change (now more obvious with the introduction of "use cache"). Use in production at your own comfort 👍