Update specific runner assignment docs

Since 15.0 users cannot convert shared runners to specific runners.

This change clarifies the workflow to explain that users must first
register their specific (project) runner before assigning it to
other projects.

doc/user/admin_area/settings/continuous_integration.md

@@ -46,7 +46,10 @@ limit on the number of [CI/CD minutes](../../../ci/pipelines/cicd_minutes.md) yo
 
 ## Enable a specific runner for multiple projects
 
-To enable a specific runner for one or more projects:
+If you have already registered a [specific runner](../../../ci/runners/runners_scope.md#specific-runners)
+you can assign that runner to other projects.
+
+To enable a specific runner for more than one project:
 
 1. On the top bar, select **Main menu > Admin**.
 1. From the left sidebar, select **Overview > Runners**.