dcb : Fix incorrect documentation for struct dcb_app

While IEEE and CEE use the same structure to store apps, the selector
and priority fields for both are different. Only the priority field is
explained, add documentation explaining how the selector field differs
for both.

cgdcbxd code shows an example of how selector fields differ.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>