Update GitLab Duo data usage docs to clarify retention

What does this MR do and why?

Makes largely stylistic updates to the data retention section of GitLab Duo's data usage docs page.

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assigned to @tichen

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	1 Message
	This merge request adds or changes documentation files. A review from the Technical Writing team before you merge is recommended. Reviews can happen after you merge.

Documentation review

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• doc/user/gitlab_duo/data_usage.md (Link to current live version)

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mentioned in commit 9f0bcd60

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