Documentation: dmaengine: Add a description of what dmatest does

Currently it can difficult to determine what dmatest does without
reading the source code. Let's add a description.

The description is taken mostly from the patch header of
commit 4a776f0a ("dmatest: Simple DMA memcpy test client").
It has been edited and updated slightly. Nevertheless the new text was
largely written by Haarvard Skinnemoen and was copied from another
patch, already committed to the kernel, which has Haarvard's SoB:
attached to it.
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20211118100952.27268-2-daniel.thompson@linaro.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/driver-api/dmaengine/dmatest.rst

@@ -6,6 +6,16 @@ Andy Shevchenko <andriy.shevchenko@linux.intel.com>
 
 This small document introduces how to test DMA drivers using dmatest module.
 
+The dmatest module tests DMA memcpy, memset, XOR and RAID6 P+Q operations using
+various lengths and various offsets into the source and destination buffers. It
+will initialize both buffers with a repeatable pattern and verify that the DMA
+engine copies the requested region and nothing more. It will also verify that
+the bytes aren't swapped around, and that the source buffer isn't modified.
+
+The dmatest module can be configured to test a specific channel. It can also
+test multiple channels at the same time, and it can start multiple threads
+competing for the same channel.
+
 .. note::
   The test suite works only on the channels that have at least one
   capability of the following: DMA_MEMCPY (memory-to-memory), DMA_MEMSET