Clarify cl_khr_external_memory_win32 import behavior

[joshqti]

Clarify the behavior when importing NT handles, and the image or buffer is from a different vendor than the OpenCL implementation.

[bashbaug]

Discussed in the April 1st memory subgroup. This may be a more general issue about cross-adapter sharing.

[bashbaug]

Discussed in the April 22nd memory subgroup. Currently, the only way this could work is with linear layout, though extensions in flight may help to make this more robust (e.g. DRM format modifiers). We will consider adding an informative note to the spec for now to reduce confusion.