Clarify expectations for feature flags and finders

Why is this change being made?

The current feature flag documentation suggests that all use of finders should be wrapped in feature flags, but since finders are tyipcally mundane and well tested, this is reworded to explicitly call out reusing complex finders in new ways. New finders are already covered by introducing complex queries.

We want to make sure we're not discouraging the use of finders since they're an important tool for isolating queries in testable units. Finders also automatically trigger database review, meaning that the queries are extensively reviewed.

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