loader: Add documentation fo loader handling of VK_surface_*KHR extensions.
loader: Add documentation fo loader handling of VK_surface_*KHR extensions.

Document the special handling og vkSurfaceKHR objects.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools