Docs: update MPLS FIB section with text from the wiki
Docs: update MPLS FIB section with text from the wiki

Change-Id: I9d903db89facc916fb5dd23d564417230e1c76e0
Signed-off-by: Neale Ranns <b9c0e9ef0223633bc2f318425f615cf5840a0be7@cisco.com>

vpp-dev/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp