Add explanation to CI default env variable (#41731)

Co-authored-by: Ben Ahmady <32935794+subatoi@users.noreply.github.com>

Author: Marcono1234

content/actions/reference/workflows-and-actions/variables.md

@@ -29,7 +29,7 @@ We strongly recommend that actions use variables to access the filesystem rather
 
 | Variable | Description |
 | ---------|------------ |
-| `CI` | Always set to `true`. |
+| `CI` | Always set to `true`. You can use this variable to differentiate when tests are being run locally or by {% data variables.product.prodname_actions %}. |
 | `GITHUB_ACTION` | The name of the action currently running, or the [`id`](/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idstepsid) of a step. For example, for an action, `__repo-owner_name-of-action-repo`.<br><br>{% data variables.product.prodname_dotcom %} removes special characters, and uses the name `__run` when the current step runs a script without an `id`. If you use the same script or action more than once in the same job, the name will include a suffix that consists of the sequence number preceded by an underscore. For example, the first script you run will have the name `__run`, and the second script will be named `__run_2`. Similarly, the second invocation of `actions/checkout` will be `actionscheckout2`. |
 | `GITHUB_ACTION_PATH` | The path where an action is located. This property is only supported in composite actions. You can use this path to change directories to where the action is located and access other files in that same repository. For example, `/home/runner/work/_actions/repo-owner/name-of-action-repo/v1`. |
 | `GITHUB_ACTION_REPOSITORY` | For a step executing an action, this is the owner and repository name of the action. For example, `actions/checkout`. |