rust: dma: clarify wording and be consistent in `coherent` nomenclature

rust: dma: clarify wording and be consistent in `coherent` nomenclature

In the kernel, `consistent` and `coherent` are used interchangeably for the
region described in this api. Stick with `coherent` nomenclature
to show that dma_alloc_coherent() is being used, in addition to improving
the clarity in the DMA mapping attributes documentation.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250602085444.1925053-2-abdiel.janulgue@gmail.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>