docs: dmaengine: add explanation for DMA_ASYNC_TX capability

docs: dmaengine: add explanation for DMA_ASYNC_TX capability

This patch replaces the TODO for DMA_ASYNC_TX in the DMA engine
provider documentation. The flag is automatically set by the DMA
framework when a device supports key asynchronous memory-to-memory
operations such as memcpy, memset, xor, pq, xor_val, and pq_val.

It must not be set by drivers directly.

Signed-off-by: Kendra Moore <kendra.j.moore3443@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250421010205.84719-1-kendra.j.moore3443@gmail.com>