ospf6d: Documentation for authentication trailer support.

ospf6d: Documentation for authentication trailer support.

Problem Statement:
=================
This commit is to add document support for OSPF6 authentication
trailer feature, which is adding support for RFC7166.

RCA:
====
NA

Fix:
====
To add detailed description for feature support.
This document caputres
Configuration CLI
Show commands
Debug commands
Clear command

That are added as part of the feature with examples.

Risk:
=====
Low

Tests Executed:
===============
NA

Signed-off-by: Abhinay Ramesh <rabhinay@vmware.com>