docs: networking: clarify intended audience of netdevices.rst

The netdevices doc is dangerously broad. At least make it clear
that it's intended for developers, not for users.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250415172653.811147-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>