Make more explicit the fact that most variables don't need editing on docker setups

deploy/env.prod.sample

@@ -4,12 +4,13 @@
 # - DJANGO_ALLOWED_HOSTS
 # - FUNKWHALE_URL
 
-# Additionaly, on non-docker setup, you'll also have to tweak/uncomment those
-# variables:
+# Additionaly, on non-docker setup **only**, you'll also have to tweak/uncomment those variables:
 # - DATABASE_URL
 # - CACHE_URL
 # - STATIC_ROOT
 # - MEDIA_ROOT
+#
+# You **don't** need to update those variables on pure docker setups.
 
 # Docker only
 # -----------