Clarify multiple metrics in docs (#2712)

Co-authored-by: Dylan Boyd <5061312+dylanjboyd@users.noreply.github.com>

docs/automatically-scaling-runners.md

@@ -186,6 +186,38 @@ spec:
     scaleDownAdjustment: 1      # The scale down runner count subtracted from the desired count
 ```
 
+**Combining Pull Driven Scaling Metrics**
+
+If a HorizontalRunnerAutoscaler is configured with a secondary metric of `TotalNumberOfQueuedAndInProgressWorkflowRuns`, then be aware that the controller will check the primary metric of `PercentageRunnersBusy` first and will only use the secondary metric to calculate the desired replica count if the primary metric returns 0 desired replicas.
+
+`PercentageRunnersBusy` metrics must appear before `TotalNumberOfQueuedAndInProgressWorkflowRuns`; otherwise, the controller will fail to process the `HorizontalRunnerAutoscaler`. A valid configuration follows.
+
+```yaml
+---
+apiVersion: actions.summerwind.dev/v1alpha1
+kind: HorizontalRunnerAutoscaler
+metadata:
+  name: example-runner-deployment-autoscaler
+spec:
+  scaleTargetRef:
+    kind: RunnerDeployment
+    # # In case the scale target is RunnerSet:
+    # kind: RunnerSet
+    name: example-runner-deployment
+  minReplicas: 1
+  maxReplicas: 5
+  metrics:
+  - type: PercentageRunnersBusy
+    scaleUpThreshold: '0.75'    # The percentage of busy runners at which the number of desired runners are re-evaluated to scale up
+    scaleDownThreshold: '0.3'   # The percentage of busy runners at which the number of desired runners are re-evaluated to scale down
+    scaleUpAdjustment: 2        # The scale up runner count added to desired count
+    scaleDownAdjustment: 1      # The scale down runner count subtracted from the desired count
+  - type: TotalNumberOfQueuedAndInProgressWorkflowRuns
+    repositoryNames:
+    # A repository name is the REPO part of `github.com/OWNER/REPO`
+    - myrepo
+```
+
 ## Webhook Driven Scaling
 
 > This feature requires controller version => [v0.20.0](https://github.com/actions/actions-runner-controller/releases/tag/v0.20.0)
@@ -431,8 +463,6 @@ The main use case for scaling from 0 is with the `HorizontalRunnerAutoscaler` ki
 
 `PercentageRunnersBusy` can't be used alone for scale-from-zero as, by its definition, it needs one or more GitHub runners to become `busy` to be able to scale. If there isn't a runner to pick up a job and enter a `busy` state then the controller will never know to provision a runner to begin with as this metric has no knowledge of the job queue and is relying on using the number of busy runners as a means for calculating the desired replica count.
 
-If a HorizontalRunnerAutoscaler is configured with a secondary metric of `TotalNumberOfQueuedAndInProgressWorkflowRuns` then be aware that the controller will check the primary metric of `PercentageRunnersBusy` first and will only use the secondary metric to calculate the desired replica count if the primary metric returns 0 desired replicas.
-
 Webhook-based autoscaling is the best option as it is relatively easy to configure and also it can scale quickly.
 
 ## Scheduled Overrides