docs: net: clarify the NAPI rules around XDP Tx

page pool and XDP should not be accessed from IRQ context
which may happen if drivers try to clean up XDP TX with
NAPI budget of 0.

Link: https://lore.kernel.org/r/20230720161323.2025379-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

docs: networking: document NAPI

Add basic documentation about NAPI. We can stop linking to the ancient
doc on the LF wiki.

Link: https://lore.kernel.org/all/20230315223044.471002-1-kuba@kernel.org/
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz> # for ctucanfd-driver.rst
Reviewed-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230322053848.198452-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>