Add zero downtime upgrade detail for production_replicas

doc/topics/autodevops/index.md

@@ -723,7 +723,7 @@ also be customized, and you can easily use a [custom buildpack](#custom-buildpac
 | `AUTO_DEVOPS_CHART_REPOSITORY_USERNAME` | From Gitlab 11.11, this variable can be used to set a username to connect to the helm repository. Defaults to no credentials. (Also set AUTO_DEVOPS_CHART_REPOSITORY_PASSWORD) |
 | `AUTO_DEVOPS_CHART_REPOSITORY_PASSWORD` | From Gitlab 11.11, this variable can be used to set a password to connect to the helm repository. Defaults to no credentials. (Also set AUTO_DEVOPS_CHART_REPOSITORY_USERNAME) |
 | `REPLICAS`                   | The number of replicas to deploy; defaults to 1.                                                                                                                                                                              |
-| `PRODUCTION_REPLICAS`        | The number of replicas to deploy in the production environment. This takes precedence over `REPLICAS`; defaults to 1.                                                                                                         |
+| `PRODUCTION_REPLICAS`        | The number of replicas to deploy in the production environment. Takes precedence over `REPLICAS` and defaults to 1. For zero downtime upgrades, set to 2 or greater.                                |
 | `CANARY_REPLICAS`            | The number of canary replicas to deploy for [Canary Deployments](../../user/project/canary_deployments.md); defaults to 1                                                                              |
 | `CANARY_PRODUCTION_REPLICAS` | The number of canary replicas to deploy for [Canary Deployments](../../user/project/canary_deployments.md) in the production environment. This takes precedence over `CANARY_REPLICAS`; defaults to 1  |
 | `ADDITIONAL_HOSTS`           | Fully qualified domain names specified as a comma-separated list that are added to the ingress hosts.                                                                                                                         |