docs: dma-api: update streaming DMA physical address constraints

Add an introductory paragraph to Part Id - Streaming DMA mappings and move
the explanation of address constraints there, because it applies to all map
functions.

Clarify that streaming DMA can be used with memory which does not meet the
addressing constraints of a device, but it may fail in that case.

Make a note about SWIOTLB and link to the detailed description of it.

Do not mention platform-dependent allocation flags. The note may mislead
device driver authors into thinking that they should poke into and try to
second-guess the DMA API implementation. They definitely shouldn't.

Remove the claim that platforms with an IOMMU may not require physically
contiguous buffers. The current implementation explicitly rejects vmalloc
addresses, regardless of IOMMU.

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/20250627101015.1600042-8-ptesarik@suse.com

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