layers: Update layer validation details doc
layers: Update layer validation details doc

Updated entry for new BufferImageCopy checks.

Change-Id: I85ea2eeacae6885fbd6729180167cbe1abe4c176

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers