Expain usage of `artifacts_server` in `gitlab.yml`

Add comment to clarify intended usage of `artifacts_server`.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/49475.

config/gitlab.yml.example

@@ -245,7 +245,7 @@ production: &base
     host: example.com
     port: 80 # Set to 443 if you serve the pages with HTTPS
     https: false # Set to true if you serve the pages with HTTPS
-    artifacts_server: true
+    artifacts_server: true # Set to false if you want to disable online view of HTML artifacts
     # external_http: ["1.1.1.1:80", "[2001::1]:80"] # If defined, enables custom domain support in GitLab Pages
     # external_https: ["1.1.1.1:443", "[2001::1]:443"] # If defined, enables custom domain and certificate support in GitLab Pages
     admin: