net: qdisc: enhance default_qdisc documentation

Aside from some lingual cleanup, point out which interfaces are not or
partly covered by this setting.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>