`cache-hit` needs to be replaced

According to #1514, cache-hit is true when the cache is restored from the full key, false when the cache is restored from a fallback key, and empty when the cache wasn't hit at all. First, please merge this documentation update, as it means most usage of this value is wrong (few people care if the fallback key was used, quite a lot of people care when the cache wasn't hit). Second, this needs to be a different API, as a Bool is inappropriate for a three part value. Instead it should be replaced with something like cache-status: found | fallbackUsed | notFound, so users can be completely clear about what's happening.

Start by reading issue #1514 and the current documentation and API behavior for cache-hit. Define the replacement status values and update the documentation so full-key hits, fallback hits, and misses are unambiguous; done means the documented API no longer relies on the boolean interpretation.