Merge branch 'artifact-name' into 'master'

Clarify artifacts:name default value

## What does this MR do?

Add the default artifact name (`artifacts`) to documentation.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

We didn't document the default artifact name.

## What are the relevant issue numbers?

## Screenshots (if relevant)


See merge request !5377

doc/ci/yaml/README.md

@@ -645,9 +645,10 @@ be available for download in the GitLab UI.
 Introduced in GitLab 8.6 and GitLab Runner v1.1.0.
 
 The `name` directive allows you to define the name of the created artifacts
-archive. That way, you can have a unique name of every archive which could be
+archive. That way, you can have a unique name for every archive which could be
 useful when you'd like to download the archive from GitLab. The `artifacts:name`
 variable can make use of any of the [predefined variables](../variables/README.md).
+The default name is `artifacts`, which becomes `artifacts.zip` when downloaded.
 
 ---