Draft: Clarify Training data on Suggested Reviewers feature

What does this MR do and why?

This MR clarifies that Training data for Suggested Reviewers feature contains public GitLab-owned project repositories under gitlab-org and gitlab-com namespaces on gitlab.com.

See slack discussion (Internal Only)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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