tools: nfdc help command

refs #3780

Change-Id: Ibd0e37bad127a38e582864fadd81afadc74baf48
diff --git a/docs/manpages/nfdc-face.rst b/docs/manpages/nfdc-face.rst
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+nfdc-face
+=========
+
+SYNOPSIS
+--------
+| nfdc face [list]
+| nfdc channel [list]
+| nfdc create [-P] <faceUri>
+| nfdc destroy <faceId|faceUri>
+
+DESCRIPTION
+-----------
+In NFD, a face is the generalization of network interface.
+It could be a physical network interface to communicate on a physical link,
+an overlay communication channel between NFD and a remote node,
+or an inter-process communication channel between NFD and a local application.
+
+The **nfdc face list** command shows a list of faces, their properties, and statistics.
+
+The **nfdc channel list** command shows a list of channels.
+Channels are listening sockets that can accept incoming connections and create new faces.
+
+The **nfdc create** command creates a new UDP or TCP face.
+
+The **nfdc destroy** command destroys an existing face.
+It has no effect if the specified face does not exist.
+
+OPTIONS
+-------
+-P
+    Creates a "permanent" rather than persistent face.
+    A persistent face is closed when a socket error occrs.
+    A permanent face is kept alive upon socket errors,
+    and is closed only upon **nfdc destroy** command.
+
+<faceUri>
+    An URI representing the remote endpoint of a face.
+    Its syntax is:
+
+    - udp[4|6]://<IP-or-host>[:<port>]
+    - tcp[4|6]://<IP-or-host>[:<port>]
+
+    When a hostname is specified, a DNS query is used to obtain the IP address.
+
+<faceId>
+    Numerical identifier of the face.
+    It is displayed in the output of **nfdc face list** and **nfdc create** commands.
+
+SEE ALSO
+--------
+nfd(1), nfdc(1)