dmaengine: Clarify cyclic transfer residue documentation

The current documentation for the residue reported in a cyclic transfer
case mentions that the reported residue should be relative to the current
period only. However the definition of DMA_RESIDUE_GRANULARITY_SEGMENT
specifies that the residue should be updated after each period for
a cyclic transfer, which is in direct contradiction.

Moreover the pcm_dmaengine common code uses the residue relative to
the whole cyclic buffer size, not one period.

Correct the residue-related documentation to reflect this.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://lore.kernel.org/r/20220331134114.703782-1-paul.kocialkowski@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>