netlink: specs: netdev: clarify the page pool API a little

netlink: specs: netdev: clarify the page pool API a little

The phrasing of the page-pool-get doc is very confusing.
It's supposed to highlight that support depends on the driver
doing its part but it sounds like orphaned page pools won't
be visible.

The description of the ifindex is completely wrong.
We move the page pool to loopback and skip the attribute if
ifindex is loopback.

Link: https://lore.kernel.org/20260104084347.5de3a537@kernel.org
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/20260104165232.710460-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>