docs: dma-api: clean up documentation of dma_map_sg()

docs: dma-api: clean up documentation of dma_map_sg()

Describe in one sentence what the function does.

Do not repeat example situations when the returned number is lower than
the number of segments on input.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250627101015.1600042-9-ptesarik@suse.com