cache-hit needs to be replaced

[jshier]

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.