Add note for some functions and fix document error

Change-Id: Ide2500c393d2a135f343263dd8c3e3f0e644efe0
diff --git a/configuration/xconnect.rst b/configuration/xconnect.rst
index a33d2b2..5bc5fc4 100644
--- a/configuration/xconnect.rst
+++ b/configuration/xconnect.rst
@@ -76,3 +76,6 @@
 
 - ``deviceId``: device ID, e.g. **of:0000000000000201**
 - ``vlanId``: VLAN ID, e.g. **94**
+
+.. caution::
+    For a given port, we should avoid using the same VLAN ID in both xconnect and interface configuration at the same time (regardless of untagged, tagged or native)