docs: remove obsolete description about threaded NAPI

docs: remove obsolete description about threaded NAPI

Commit 2677010e7793 ("Add support to set NAPI threaded for individual
NAPI") introduced threaded NAPI configuration per individual NAPI
instance, however obsolete description that threaded NAPI is per device
has remained.

Remove the old description and clarify that only NAPI instances running
in threaded mode spawn kernel threads by changing "Each NAPI instance"
to "Each threaded NAPI instance".

Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Link: https://patch.msgid.link/20250829064857.51503-1-enjuk@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>