Add info regarding opt-in attributes

[Luc1412]

The docs should include a warning/info for attributes that require opt in. The user should be also educated on how to opt-in.

This includes:

• item_preview_hero_path
• display_asset_path
• definition_path
• path
• gameplay_tags
• shop_history

[trevorflahardy]

As a side note, this would best be served as a custom sphinx directive for the sake of removing repetition.