net: sched: Cleanup PIE comments

Fix incorrect comment reported by Norbert Kiesel. Edit another comment to add
more details. Also add references to algorithm (IETF draft and paper) to top of
file.

Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>
CC: Mythili Prabhu <mysuryan@cisco.com>
CC: Norbert Kiesel <nkiesel@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>