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>