dmaengine: xilinx: xdma: Clarify kdoc in XDMA driver

Clarify the kernel doc of xdma_fill_descs(), especially how big chunks
will be handled.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://lore.kernel.org/stable/20240327-digigram-xdma-fixes-v1-3-45f4a52c0283%40bootlin.com
Link: https://lore.kernel.org/r/20240327-digigram-xdma-fixes-v1-3-45f4a52c0283@bootlin.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>