Document why FF rollouts with percentage of actors is preferred over percentage of time

As a follow-up to #327117 (closed):

Document why "percentage of time" rollout is discouraged and we should use "percentage of actors" instead. For percentage of > time to be used correctly we need to ensure that the feature works correctly in between feature checks. If we use > Feature.enabled? multiple times in the code, the "% of time" has good chance of introducing inconsistent behaviour.