Documentation: update networking/page_pool.rst

mainline inclusion
from mainline-v5.18-rc1
commit a3dd9828
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8BNYE
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a3dd98281b9f265c7b89cb0c7a91739bff2e6506



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Add the new stats API, kernel config parameter, and stats structure
information to the page_pool documentation.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: David S. Miller <davem@davemloft.net>