add documentation for the cubin header check and explain the reason

doc/src/Build_extras.rst

@@ -132,6 +132,10 @@ Thus the GPU_ARCH setting is merely an optimization, to have code for
 the preferred GPU architecture directly included rather than having to wait
 for the JIT compiler of the CUDA driver to translate it.
 
+When building with CMake, you **must NOT** build the GPU library in ``lib/gpu``
+using the traditional build procedure. CMake will detect files generated by that
+process and will terminate with an error and a suggestion for how to remove them.
+
 If you are compiling with HIP, note that before running CMake you will have to
 set appropriate environment variables. Some variables such as
 :code:`HCC_AMDGPU_TARGET` or :code:`CUDA_PATH` are necessary for :code:`hipcc`