doc: update mpls per interface command

Update mpls per interface command information.

Signed-off-by: Philippe Guibert <[email protected]>

doc/user/zebra.rst

@@ -156,12 +156,15 @@ Standard Commands
    Set description for the interface.
 
 
-.. clicmd:: mpls enable
+.. clicmd:: mpls <enable|disable|auto>
 
-   Enable or disable mpls kernel processing on the interface, for linux.  Interfaces
+   Choose mpls kernel processing value on the interface, for linux. Interfaces
    configured with mpls will not automatically turn on if mpls kernel modules do not
-   happen to be loaded.  This command will fail on 3.X linux kernels and does not
-   work on non-linux systems at all.
+   happen to be loaded. This command will fail on 3.X linux kernels and does not
+   work on non-linux systems at all. 'enable' and 'disable' will respectively turn
+   on and off mpls on the given interface. The 'auto' mode will enable mpls on the
+   given interface, if an mpls entry or a labeled route happens to be installed and
+   needs mpls processing to be turned on.
 
 .. clicmd:: multicast