Update FF guidance with additional examples

Why is this change being made?

As part of the incident and the following review, a slack discussion suggested revising the examples provided for feature flag use to include destructive data operations and the use of finders to be explicitly called out.

Relates to: Review and revise feature flag guidance for Man... (gitlab-org/gitlab#418564 - closed)

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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