Configuring Pull Request queries should have a sample query or documentation link

[binderjoe]

Testing #6989

Steps to Reproduce

1. Click Configure ... in the Pull Requests pane.
2. Configure Queries.
3. The settings for the queries opens:
   
4. Clicking Edit in Settings.json opens Settings.json, but there aren't any examples to modify.
5. Similarly, clicking the GitHub search syntax link opens a generic documentation page that doesn't describe how to format the queries as a VS Code setting.

Problem

It took quite a bit of searching to find a good example of how to author custom PR queries in Settings.json.

Suggestion

I suggest that we:

• Add a documentation page with an example of how to author custom PR queries in Settings.json, or
• Include an example in the Settings UI that users