tools: delete -p in ndnsec-export/import help

ndnsec-export and ndnsec-import man pages are also updated.

refs #4313

Change-Id: I1ba41123c5fb46f2e408da3d55dfe9bef56d5b27
diff --git a/docs/manpages/ndnsec-import.rst b/docs/manpages/ndnsec-import.rst
index 5b61f59..fbc8bc9 100644
--- a/docs/manpages/ndnsec-import.rst
+++ b/docs/manpages/ndnsec-import.rst
@@ -1,33 +1,33 @@
 ndnsec-import
 =============
 
-``ndnsec-import`` is a tool to import an identity's security data that is prepared by
-``ndnsec-export``.
-
 Usage
 -----
 
 ::
 
-    $ ndnsec-import [-h] [-p] input
+    $ ndnsec-import [-h] input
 
 Description
 -----------
 
-``ndnsec-import`` read data from ``input``. It will ask for password if the input contains private
-key. If ``input`` is ``-``, security data will be read from standard input.
+``ndnsec-import`` imports a certificate and private key from a file created by ``ndnsec-export``. It
+will ask for the passphrase used to encrypt the private key.
 
 Options
 -------
 
-``-p``
-  Indicates the imported data containing private key. A password will be asked for data encryption.
+``-h``
+  Print a help message.
+
+``input``
+  Read from an input file. Specify ``-`` to read from the standard input.
 
 Examples
 --------
 
-Import an identity's security data including private key:
+Import a certificate and private key from a file:
 
 ::
 
-    $ ndnsec-import -p input_file
+    $ ndnsec-import alice.ndnkey