docs: dma-api: remove remnants of PCI DMA API

docs: dma-api: remove remnants of PCI DMA API

The wording sometimes suggests there are multiple functions for an
operation. This was in fact the case before PCI DMA API was removed, but
since there is only one API now, the documentation has become confusing.

To improve readability:

* Remove implicit references to the PCI DMA API (plurals, use of "both",
  etc.)

* Where possible, refer to an actual function rather than a more generic
  description of the operation.

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/20250627101015.1600042-4-ptesarik@suse.com