You do not need to write your own book about how Group Policy works in your environment, but the further you get into your journey of implementing GPOs in your organization, the more you will wish that you had followed some good documentation practices from the start. We already discussed naming GPOs according to the settings contained within, as this good practice alone will help tremendously when looking back on these GPOs a year down the road.

Additionally, there are some other things that you could be doing on a regular basis whenever you create a new policy that will help you to flesh out documentation for your AD environment. These items will be particularly useful for any other administrators that log into the domain and attempt to diagnose or otherwise figure out the purposes behind your policies.