Document which feature flag can be enabled where

Overview

In !2811 (merged) we added the functionality that runner admin can define feature flags inside of the config.toml and inside in !2805 (merged) we added a future flag that can only be defined inside of the config.toml and not inside of the job variable.

We also have FF_USE_FASTZIP where it can only be defined as a variable.

We need to document this process for users so they are aware where and how they can define each feature flag.