Clarify wording for runs-on syntax (#12504)

Co-authored-by: Edward Thomson <ethomson@github.com>
Co-authored-by: Sarah Edwards <skedwards88@github.com>

Author: 3 people

content/actions/learn-github-actions/workflow-syntax-for-github-actions.md

@@ -465,7 +465,7 @@ In this example, `job3` uses the `always()` conditional expression so that it al
 
 ## `jobs.<job_id>.runs-on`
 
-**Required**. The type of machine to run the job on. The machine can be either a {% data variables.product.prodname_dotcom %}-hosted runner or a self-hosted runner.
+**Required**. The type of machine to run the job on. The machine can be either a {% data variables.product.prodname_dotcom %}-hosted runner or a self-hosted runner. You can provide `runs-on` as a single string or as an array of strings.
 
 {% ifversion ghae %}
 ### {% data variables.actions.hosted_runner %}s

data/reusables/github-actions/self-hosted-runner-labels-runs-on.md

@@ -1,3 +1,5 @@
 To specify a self-hosted runner for your job, configure `runs-on` in your workflow file with self-hosted runner labels.
 
-All self-hosted runners have the `self-hosted` label. Using only this label will select any self-hosted runner. To select runners that meet certain criteria, such as operating system or architecture, provide an array of labels that begins with `self-hosted` (this must be listed first) and then includes additional labels as needed.
+All self-hosted runners have the `self-hosted` label. Using only this label will select any self-hosted runner. To select runners that meet certain criteria, such as operating system or architecture, we recommend providing an array of labels that begins with `self-hosted` (this must be listed first) and then includes additional labels as needed. When you specify an array of labels, jobs will be queued on runners that have all the labels that you specify.
+
+Although the `self-hosted` label is not required, we strongly recommend specifying it when using self-hosted runners to ensure that your job does not unintentionally specify any current or future {% data variables.product.prodname_dotcom %}-hosted runners.