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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

Documentation/driver-api/dmaengine/provider.rst

···
 
 - DMA_ASYNC_TX
 
-  - Must not be set by the device, and will be set by the framework
-    if needed
+  - The device supports asynchronous memory-to-memory operations,
+    including memcpy, memset, xor, pq, xor_val, and pq_val.
 
-  - TODO: What is it about?
+  - This capability is automatically set by the DMA engine
+    framework and must not be configured manually by device
+    drivers.
 
 - DMA_SLAVE