docs: net: document netdev notifier expectations

acquiring the instance lock themselves, while the ``netif_xxx`` functions

assume that the driver has already acquired the instance lock.

Notifiers and netdev instance lock

==================================

For device drivers that implement shaping or queue management APIs,

some of the notifiers (``enum netdev_cmd``) are running under the netdev

instance lock.

For devices with locked ops, currently only the following notifiers are

running under the lock:

* ``NETDEV_REGISTER``

* ``NETDEV_UP``

The following notifiers are running without the lock:

* ``NETDEV_UNREGISTER``

There are no clear expectations for the remaining notifiers. Notifiers not on

the list may run with or without the instance lock, potentially even invoking

the same notifier type with and without the lock from different code paths.

The goal is to eventually ensure that all (or most, with a few documented

exceptions) notifiers run under the instance lock. Please extend this

documentation whenever you make explicit assumption about lock being held

from a notifier.

NETDEV_INTERNAL symbol namespace

================================