loopback: add missing interface description

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diff --git a/docs/interfaces/loopback.rst b/docs/interfaces/loopback.rst
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+.. _loopback-interface:
+
+########
+Loopback
+########
+
+The loopback networking interface is a virtual network device implemented
+entirely in software. All traffic sent to it "loops back" and just targets
+services on your local machine.
+
+.. note:: There can only be one loopback ``lo`` interface on the system. If
+   you need multiple interfaces, please use the :ref:`dummy-interface`
+   interface type.
+
+Configuration
+=============
+
+Address
+-------
+
+.. cfgcmd:: set interfaces loopback lo address <address>
+
+   Configure Loopback interface `lo` with one or more interface addresses.
+
+   * **address** can be specified multiple times as IPv4 and/or IPv6 address,
+     e.g. 192.0.2.1/24 and/or 2001:db8::1/64
+
+Link Administration
+-------------------
+
+.. cfgcmd:: set interfaces loopback lo description <description>
+
+   Assign given `<description>` to interface `lo`. Description will also be
+   passed to SNMP monitoring systems.
+
+Operation
+=========
+
+.. opcmd:: show interfaces loopback
+
+   Show brief interface information.
+
+   .. code-block:: none
+
+     vyos@vyos:~$ show interfaces loopback
+     Codes: S - State, L - Link, u - Up, D - Down, A - Admin Down
+     Interface        IP Address                        S/L  Description
+     ---------        ----------                        ---  -----------
+     lo               127.0.0.1/8                       u/u
+                      ::1/128
+
+.. opcmd:: show interfaces loopback lo
+
+   Show detailed information on given loopback interface `lo`.
+
+   .. code-block:: none
+
+     vyos@vyos:~$ show interfaces ethernet eth0
+     lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
+         link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
+         inet 127.0.0.1/8 scope host lo
+            valid_lft forever preferred_lft forever
+         inet6 ::1/128 scope host
+            valid_lft forever preferred_lft forever
+
+         RX:  bytes    packets     errors    dropped    overrun      mcast
+                300          6          0          0          0          0
+         TX:  bytes    packets     errors    dropped    carrier collisions
+                300          6          0          0          0          0