Add docs about memcache tuning

[weeco]

Memcache can have a great positive impact on performance. If setup incorrectly you won't benefit much from it. It's not easy to tell whether you are properly utilizing memcache. For instance:

1. Is your Memcache cluster oversized / undersized?
2. Should you reserve some memcache storage for the different components? a) Is it more important to cache Query Frontend stuff than index writes? b) If yes, does it make sense to have multiple memcache clusters, each component has it's own to make sure they have dedicated storage sizes? Or is it possible to configure memcache so that each component can have it's own dedicated storage within a single cluster

Slack discussion: https://cloud-native.slack.com/archives/CCYDASBLP/p1574096262229600