net: page_pool: hide page_pool_release_page()

There seems to be no user calling page_pool_release_page()
for legit reasons, all the users simply haven't been converted
to skb-based recycling, yet. Previous changes converted them.
Update the docs, and unexport the function.

Link: https://lore.kernel.org/r/20230720010409.1967072-4-kuba@kernel.org


Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>