Merge branch 'docs-redis-ha-upgrade' into 'master'

Add docs about upgrading Redis HA setup

See merge request gitlab-org/omnibus-gitlab!3815
8 jobs for master in 63 minutes and 53 seconds
Status Job ID Name Coverage
  Prepare Assets
passed #400011840
docker
fetch-assets

00:04:04

 
  Trigger Package
passed #400011841
triggered-packages
Trigger:package

00:27:18

 
  Trigger Docker
passed #400011842
docker
Trigger:gitlab-docker

00:10:46

passed #400011843
docker
Trigger:qa-docker

00:12:13

 
  Trigger Ha Validate
manual #400011844
allowed to fail manual
Trigger:ha-validate
 
  Trigger Qa
passed #400011845
Trigger:qa-test

00:20:16

 
  Metrics
manual #400011846
docker allowed to fail manual
Trigger:measure-memory
 
  Notification Fail
skipped #400011847
notify:slack-fail:scheduled-master