Add to feature flag docs to manually enable when used

What does this MR do and why?

This MR adds a line to the feature flag docs to highly recommend that we manually add enabled feature flags to our tests rather than relying on the default on convention.

Doing so makes it far clearer what environment the tests require to pass and makes searching for tests which rely on the feature flag that much easier.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.