Cache docs improvement and yaml config typo fix.

[BradErz]

We were trying to add the cortex-query-frontend in front of our existing thanos setup to improve response time of queries and these are the issues we came across.

1. The chunk/cache.Config is currently defaul_validity instead of default_validity.
2. The docs for the query-frontend were not so clear, but from checking the code either cache.redis.Expiration/cache.memcache.Expiration or cache.default_validity need to be set before the query-frontend actually saves objects in memcache/redis. Whats the best "fix" for this? Clearer docs describing this behaviour? setting a sensible default for default_validity? or both?