docs: dmaengine: document ownership of the descriptor flags field

The flags field of struct dma_async_tx_descriptor has carried a TODO
since the provider documentation was added in 2014, asking whether a
driver may modify the field itself or whether it must always hold the
flags passed in the arguments.

The field belongs to the client. dma_async_tx_descriptor_init() does
not touch it, so the provider has to set it, and providers store the
flags they were given verbatim; vchan_tx_prep() does this for every
driver built on top of virt-dma. Afterwards providers only read the
field: the DMA_PREP_* bits are consumed while the descriptor is being
prepared, and DMA_CTRL_ACK and DMA_CTRL_REUSE are tested with
async_tx_test_ack() and dmaengine_desc_test_reuse() before a
descriptor is recycled or freed.

Once a descriptor has been prepared, nothing in the tree changes the
field except the client helpers in include/linux/dmaengine.h:
async_tx_ack(), async_tx_clear_ack(), dmaengine_desc_set_reuse() and
dmaengine_desc_clear_reuse().

Document that rule in place of the TODO.

Signed-off-by: Bogdan Mahomet <bogdan.mahomet.dev@gmail.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260822001536.29269-1-bogdan.mahomet.dev@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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