Update Auto-Devops docs to mention new REPLICAS env var

changelogs/unreleased/41436-use-simpler-env-vars-for-auto-devops-replicas.yml

+---
+title: 'Introduce simpler env vars for auto devops REPLICAS and CANARY_REPLICAS #41436'
+merge_request: 18036
+author:
+type: added

doc/topics/autodevops/index.md

@@ -459,13 +459,15 @@ also be customized, and you can easily use a [custom buildpack](#custom-buildpac
 | ------------                 | ---------------                                                                                                                                                                                                               |
 | `AUTO_DEVOPS_DOMAIN`         | The [Auto DevOps domain](#auto-devops-domain); by default set automatically by the [Auto DevOps setting](#enabling-auto-devops).                                                                                              |
 | `AUTO_DEVOPS_CHART`          | The Helm Chart used to deploy your apps; defaults to the one [provided by GitLab](https://gitlab.com/charts/charts.gitlab.io/tree/master/charts/auto-deploy-app).                                                             |
-| `PRODUCTION_REPLICAS`       | The number of replicas to deploy in the production environment; defaults to 1. |
-| `CANARY_PRODUCTION_REPLICAS`| The number of canary replicas to deploy for [Canary Deployments](https://docs.gitlab.com/ee/user/project/canary_deployments.html) in the production environment. |
+| `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.                                                                                                         |
+| `CANARY_REPLICAS`            | The number of canary replicas to deploy for [Canary Deployments](https://docs.gitlab.com/ee/user/project/canary_deployments.html); defaults to 1                                                                              |
+| `CANARY_PRODUCTION_REPLICAS` | The number of canary replicas to deploy for [Canary Deployments](https://docs.gitlab.com/ee/user/project/canary_deployments.html) in the production environment. This takes precedence over `CANARY_REPLICAS`; defaults to 1  |
 | `POSTGRES_ENABLED`           | Whether PostgreSQL is enabled; defaults to `"true"`. Set to `false` to disable the automatic deployment of PostgreSQL.                                                                                                        |
 | `POSTGRES_USER`              | The PostgreSQL user; defaults to `user`. Set it to use a custom username.                                                                                                                                                     |
 | `POSTGRES_PASSWORD`          | The PostgreSQL password; defaults to `testing-password`. Set it to use a custom password.                                                                                                                                     |
 | `POSTGRES_DB`                | The PostgreSQL database name; defaults to the value of [`$CI_ENVIRONMENT_SLUG`](../../ci/variables/README.md#predefined-variables-environment-variables). Set it to use a custom database name.                               |
-| `BUILDPACK_URL`  | The buildpack's full URL. It can point to either Git repositories or a tarball URL. For Git repositories, it is possible to point to a specific `ref`, for example `https://github.com/heroku/heroku-buildpack-ruby.git#v142`|
+| `BUILDPACK_URL`              | The buildpack's full URL. It can point to either Git repositories or a tarball URL. For Git repositories, it is possible to point to a specific `ref`, for example `https://github.com/heroku/heroku-buildpack-ruby.git#v142` |
 
 TIP: **Tip:**
 Set up the replica variables using a
@@ -496,8 +498,9 @@ The general rule is: `TRACK_ENV_REPLICAS`. Where:
 That way, you can define your own `TRACK_ENV_REPLICAS` variables with which
 you will be able to scale the pod's replicas easily.
 
-In the example below, the environment's name is `qa` which would result in
-looking for the `QA_REPLICAS` environment variable:
+In the example below, the environment's name is `qa` and it deploys the track
+`foo` which would result in looking for the `FOO_QA_REPLICAS` environment
+variable:
 
 ```yaml
 QA testing:
@@ -505,11 +508,11 @@ QA testing:
   environment:
     name: qa
   script:
-  - deploy qa
+  - deploy foo
 ```
 
-If, in addition, there was also a `track: foo` defined in the application's Helm
-chart, like:
+The track `foo` being referenced would also need to be defined in the
+application's Helm chart, like:
 
 ```yaml
 replicaCount: 1
@@ -531,8 +534,6 @@ service:
   internalPort: 5000
 ```
 
-then the environment variable would be `FOO_QA_REPLICAS`.
-
 ## Currently supported languages
 
 NOTE: **Note:**