page_pool: fix &page_pool_params kdoc issues

After the tagged commit, @netdev got documented twice and the kdoc
script didn't notice that. Remove the second description added later
and move the initial one according to the field position.

After merging commit 5f8e4007 ("kernel-doc: fix
struct_group_tagged() parsing"), kdoc requires to describe struct
groups as well. &page_pool_params has 2 struct groups which
generated new warnings, describe them to resolve this.

Fixes: 403f11ac

 ("page_pool: don't use driver-set flags field directly")
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/r/20240524112859.2757403-1-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>