Any Documentation For the ReadOnlyTargetRules in Build.cs?

You can find information in UBT source code:

https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Programs/UnrealBuildTool/Configuration/UEBuildTarget.cs#L864

You can also find Rules in

https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Programs/UnrealBuildTool/Configuration/ModuleRules.cs