Backport doc changes for High Availability/Redis to CE
https://gitlab.com/gitlab-org/gitlab-ee/commit/163d2c4d35fc63adb26737b5217f2e3c7900be7f needs to be backported to CE.
Edited by Mark Fletcher
https://gitlab.com/gitlab-org/gitlab-ee/commit/163d2c4d35fc63adb26737b5217f2e3c7900be7f needs to be backported to CE.