DMA-API: Update dma_pool_create ()and dma_pool_alloc() descriptions

Use "boundary" to be more descriptive than "alloc" in the dma_pool_create()
documentation.

Replace "SLAB_KERNEL" and "SLAB_ATOMIC" with the correct "GFP_KERNEL" and
"GFP_ATOMIC."

[bhelgaas: changelog]
Signed-off-by: Gioh Kim <gioh.kim@lge.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>