dmaengine: Mark struct dma_slave_caps kernel-doc correctly, clarify

struct dma_slave_caps documentation omitted the correct kernel-doc
opening comment mark.

Document byte granularity and interpretation of the src/dst_addr_widths
bit flag fields used by struct dma_slave_caps and struct dma_device.

Add punctuation to their "directions" member documentations, and cleanup
wording of the description.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>