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/[e-mail]
Reviewed-by: Donald Hunter <[e-mail]>
Acked-by: Jesper Dangaard Brouer <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/specs/netdev.yaml

···
         name: ifindex
         doc: |
           ifindex of the netdev to which the pool belongs.
-          May be reported as 0 if the page pool was allocated for a netdev
+          May not be reported if the page pool was allocated for a netdev
           which got destroyed already (page pools may outlast their netdevs
           because they wait for all memory to be returned).
         type: u32
···
       name: page-pool-get
       doc: |
         Get / dump information about Page Pools.
-        (Only Page Pools associated with a net_device can be listed.)
+        Only Page Pools associated by the driver with a net_device
+        can be listed. ifindex will not be reported if the net_device
+        no longer exists.
       attribute-set: page-pool
       do:
         request: