[Command-Reference] Add CLI docs for IP interface loopback action

[liorghub]

HLD: sonic-net/SONiC#1006

What I did

Add CLIs document for IP intefeature, CLI PR: #2192

How I did it

How to verify it

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

Config command

config interface loopback-action <interface-name> <action>
config interface loopback-action Ethernet0 drop

Show command

root@sonic:~# show ip interfaces loopback-action
Interface     Action      
------------  ----------  
Ethernet232   drop
Vlan100       forward    

[prsunny]

Since 'show' is part of 'ip' sub-command, should we have this as 'config interface ip loopback-action ...'

[liorghub]

Done. I am closing his PR, those changes will be part of PR #2192

[liat-grozovik]

@liorghub i suggest that the command reference will be update on the same PR of the code changes. So, lets close this one and add the .md file changes to #2216