docs: net: page_pool: document PP_FLAG_DMA_SYNC_DEV parameters

Using PP_FLAG_DMA_SYNC_DEV is a bit confusing. It was perhaps
more obvious when it was introduced but the page pool use
has grown beyond XDP and beyond packet-per-page so now
making the heads and tails out of this feature is not
trivial.

Obviously making the API more user friendly would be
a better fix, but until someone steps up to do that
let's at least document what the parameters are.

Relevant discussion in the first Link.

Link: https://lore.kernel.org/all/20230731114427.0da1f73b@kernel.org/


Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Link: https://lore.kernel.org/r/20230802161821.3621985-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>