docs: Extending documentation and manpages

Refs: #1462

Change-Id: If33a0f3c623daf3502ed301bcdc63892779463e3
diff --git a/docs/manpages/ndnsec-get-default.rst b/docs/manpages/ndnsec-get-default.rst
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+ndnsec-get-default
+==================
+
+``ndnsec-get-default`` is a tool to display the default setting of a particular entity.
+
+Usage
+-----
+
+::
+
+    $ ndnsec-get-default [-h] [-kc] [-i identity|-K key] [-q]
+
+Description
+-----------
+
+Given a particular entity, ``ndnsec-get-default`` can display its default setting as specified in
+options. If ``identity`` is specified, the given entity becomes the identity. If ``key`` is
+specified, the given identity becomes the key. If no entity is specified, the command will take the
+system default identity as the given entity.
+
+Options
+-------
+
+``-k``
+  Display the given entity's default key name.
+
+``-c``
+  Display the given entity's default certificate name.
+
+``-i identity``
+  Display default setting of the ``identity``
+
+``-K key``
+  Display default setting of the ``key``.
+
+``-q``
+  Disable trailling new line character.
+
+Examples
+--------
+
+Display an identity's default key name.
+
+::
+
+    $ ndnsec-get-default -k -i /ndn/test/alice
+    /ndn/test/alice/ksk-1394129695025
+
+Display an identity's default certificate name.
+
+::
+
+    $ ndnsec-get-default -c -i /ndn/test/alice
+    /ndn/test/KEY/alice/ksk-1394129695025/ID-CERT/%FD%01D%98%9A%F2%3F
+
+Display a key's default certificate name.
+
+::
+
+    $ ndnsec-get-default -c -K /ndn/test/alice/ksk-1394129695025
+    /ndn/test/KEY/alice/ksk-1394129695025/ID-CERT/%FD%01D%98%9A%F2%3F