Update GitLab external service docs about new defaults and settings

Created by: tsenart

As part of #2025 (closed), GitLab's external service configuration changed in the following ways:

1. The default projectQuery value is now ["none"], similar to what was done for GitHub's repositoryQuery
2. There's a new projects list setting similar to GitHub's repos setting.
3. There's a new exclude list setting similar to GitHub's exclude setting.

The documentation for GitLab needs to be updated accordingly. @ryan-blunden agreed to help with this to make the voice and form consistent.