net: add documentation for BQL helpers

Provide a kernel-doc comment documentation for the BQL helpers:
- netdev_sent_queue
- netdev_completed_queue
- netdev_reset_queue

Similarly to how it is done for the other functions, the documentation
only covers the function operating on struct net_device and not struct
netdev_queue.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>