docs: dmaengine: clarify ordering rules around cookie completion

Sashiko pointed in [1] about the cookie completion order. It is good to
Document that for ensuring that dma buffer is flushed and no stale data
is seeing by consumer, we need to make sure such action is taken before
drivers invoke dma cookie completion.

[1]: https://lore.kernel.org/r/20260917072507.5BDB61F000FF@smtp.kernel.org

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://patch.msgid.link/20261009093745.539924-1-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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