Update file competitor-discussion-guidelines.md

Update to the Guidelines on public discussion of competitor product features page of the handbook.

Changes are being made to clarify the use of, and reference to, competitor product screenshots, specifically when used in GitLab documentation, as the guidelines were not clear. This will help the technical writing team when deciding whether or not to use competitor product screenshots in the documentation.

The change will also clarify use of competitor products screenshots that include third-party logos.

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