Add QoS config update test case in design doc (#892)

Author: qiluo-msft

doc/config-generic-update-rollback/SONiC_Generic_Config_Update_and_Rollback_Design.md

@@ -922,6 +922,7 @@ N/A
 | 28        | Updating port-channel interfaces configs. |
 | 29        | Updating loopback interfaces configs. |
 | 30        | Updating BGP prefix hijack configs. |
+| 31        | Updating QoS headroom pool and buffer pool size. |
 
 ## 9.2 Unit Tests for Checkpoint
 | Test Case | Description |