Doc fix on QoS. PCP takes priority over DSCP for tagged IP packets

This is how it works and how we want it to be by default.
No code change, just doc change.

[skip ci]

doc/ChangeLog.md

@@ -10,6 +10,9 @@ All notable changes to the project are documented in this file.
 addresses for interfaces!  For details, see below issue #680.
 
 ### Changes
+- Correcting documentation on QoS. For packets containing both a VLAN
+  tag and an IP header, PCP priority takes precedence over DSCP
+  priority (not vice versa).
 - Update CONTRIBUTING.md for scaling core team and helping external
   contributors understand the development process, issue #672
 - OSPF: Add limitation to only allow one interface per area

doc/qos.md

@@ -54,8 +54,8 @@ significant bits of the DSCP is used to select the queue:
 |   6 | 48-55 | ⇒ |     6 |     25 |
 |   7 | 56-63 | ⇒ |     7 |     33 |
 
-For packets containing both a VLAN tag and an IP header, DSCP priority
-takes precedence over PCP priority. In cases where neither are
+For packets containing both a VLAN tag and an IP header, PCP priority
+takes precedence over DSCP priority. In cases where neither are
 available, packets are always assigned to queue 0.
 
 Each port's set of 8 egress queues operate on a Weighted Round Robin