Improve Elasticsearch documentation and set up experience

• Before a huge bulk indexing, we recommend disabling refreshing and replication. Let's make appropriate rake task instead! https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2401#note_34688518

• Do we also want to talk about sidekiq queues, throttling, dedicated sidekiq runners, etc, here? These are all things GitLab.com needs to do to make elasticsearch work, so we should share with the community as much as possible. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2401#note_34689991

• Setting number_of_replicas to 1 will always make one node cluster "yellow". See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2401#note_34895437