doc: seamless-resolve documentation

FEC entry resolution over seamless MPLS networks

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Dmytro Shytyi <dmytro.shytyi@6wind.com>

doc/user/zebra.rst

@@ -675,6 +675,17 @@ presence of the entry.
    21     Static       10.125.0.2  IPv4 Explicit Null
 
 
+FEC entry resolution over seamless MPLS networks
+------------------------------------------------
+
+When a FEC entry is registered, a BGP labeled route may not be 
+available to create an LSP entry. This is the case when trying
+to forge an additional LSP path with BGP, on top of an IGP LSP path.
+The following command authorises the creation of an additional
+BGP labeled LSP path over an IGP LSP path:
+
+.. clicmd:: mpls fec seamless-resolve
+
 .. _zebra-srv6:
 
 Segment-Routing IPv6