add faq answer on getinstanceprivatekeys cache clear requirement

[e-five256]

this has come up quite often as it's a gotcha when setting up a new instance that the private keys are cached and don't clear when updated

ideally the fix might be to realize when the cache is invalid or clear it as part of the key update command but I'm not sure how large of an undertaking / the risks of that, so adding that until someone smarter than I can take a look at it