documentation: networking: Add NAPI config

Document the existence of persistent per-NAPI configuration space and
the API that drivers can opt into.

Update stale documentation which suggested that NAPI IDs cannot be
queried from userspace.

Signed-off-by: Joe Damato <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/napi.rst

···
 a configuration of 1 ``rx``, 1 ``tx`` and 1 ``combined`` channel is expected
 to utilize 3 interrupts, 2 Rx and 2 Tx queues.
 
+Persistent NAPI config
+----------------------
+
+Drivers often allocate and free NAPI instances dynamically. This leads to loss
+of NAPI-related user configuration each time NAPI instances are reallocated.
+The netif_napi_add_config() API prevents this loss of configuration by
+associating each NAPI instance with a persistent NAPI configuration based on
+a driver defined index value, like a queue number.
+
+Using this API allows for persistent NAPI IDs (among other settings), which can
+be beneficial to userspace programs using ``SO_INCOMING_NAPI_ID``. See the
+sections below for other NAPI configuration settings.
+
+Drivers should try to use netif_napi_add_config() whenever possible.
+
 User API
 ========
 
 User interactions with NAPI depend on NAPI instance ID. The instance IDs
 are only visible to the user thru the ``SO_INCOMING_NAPI_ID`` socket option.
-It's not currently possible to query IDs used by a given device.
+
+Users can query NAPI IDs for a device or device queue using netlink. This can
+be done programmatically in a user application or by using a script included in
+the kernel source tree: ``tools/net/ynl/pyynl/cli.py``.
+
+For example, using the script to dump all of the queues for a device (which
+will reveal each queue's NAPI ID):
+
+.. code-block:: bash
+
+   $ kernel-source/tools/net/ynl/pyynl/cli.py \
+             --spec Documentation/netlink/specs/netdev.yaml \
+             --dump queue-get \
+             --json='{"ifindex": 2}'
+
+See ``Documentation/netlink/specs/netdev.yaml`` for more details on
+available operations and attributes.
 
 Software IRQ coalescing
 -----------------------