Update unstable_cache docs for revalidate option (#62689)
Clarifies what passing false or undefined to the revalidate property
option of unstable_cache does.
Context: I didn't want my DB query to be cached at all, but I did use
this API because I wanted to refresh a component's data after submitting
a server action. I thought I had to pass 0, or false to get it to not
cache at all.
Co-authored-by: Sam Ko <sam@vercel.com>
