Documentation: dmaengine: clean up description of dmatest usage

Fix the formatting of the multi-channel test usage example. Call out
the note about parameter ordering and add detail on the settings of
parameters for the new version of dmatest.

Fixes: f80f9988 ("dmaengine: Documentation: Add documentation for multi chan testing")
Signed-off-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>