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>

Author: kuba-moo

Documentation/networking/napi.rst

@@ -65,15 +65,16 @@ argument - drivers can process completions for any number of Tx
 packets but should only process up to ``budget`` number of
 Rx packets. Rx processing is usually much more expensive.
 
-In other words, it is recommended to ignore the budget argument when
-performing TX buffer reclamation to ensure that the reclamation is not
-arbitrarily bounded; however, it is required to honor the budget argument
-for RX processing.
+In other words for Rx processing the ``budget`` argument limits how many
+packets driver can process in a single poll. Rx specific APIs like page
+pool or XDP cannot be used at all when ``budget`` is 0.
+skb Tx processing should happen regardless of the ``budget``, but if
+the argument is 0 driver cannot call any XDP (or page pool) APIs.
 
 .. warning::
 
-   The ``budget`` argument may be 0 if core tries to only process Tx completions
-   and no Rx packets.
+   The ``budget`` argument may be 0 if core tries to only process
+   skb Tx completions and no Rx or XDP packets.
 
 The poll method returns the amount of work done. If the driver still
 has outstanding work to do (e.g. ``budget`` was exhausted)