include/linux/gfp.h: further document GFP_DMA32

kmalloc(..., GFP_DMA32) does not return DMA32 memory because the DMA32
kmalloc cache array is not implemented.  (Reason: there is no such user
in kernel).

Put a short comment about this so people can understand this by reading
the comment.

[1] https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031696.html

Link: https://lkml.kernel.org/r/20211207093610.6406-1-miles.chen@mediatek.com
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>