docs: page_pool: drop the mention of the legacy stats API

docs: page_pool: drop the mention of the legacy stats API

The Netlink support for querying page pool stats has been
proven out in production, let's remove the mention of the
helper meant for dumping page pool stats into ethtool -S
from the docs.

Call out in the kdoc that this API is deprecated.
Some drivers may not be able to use the Netlink API
(if page pool is shared across netdevs). So the old API
is not _completely_ dead. But we shouldn't advertise it.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260526155722.2790742-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>