docs: ndnsec man pages improvements

 * Add a "See Also" section to most pages
 * Streamline/clarify some sentences
 * Refresh a few examples

Change-Id: I3e0b62ad01c06faac632a120cde22a8da472426e
diff --git a/docs/manpages/ndnsec.rst b/docs/manpages/ndnsec.rst
index d65c3a7..edf948a 100644
--- a/docs/manpages/ndnsec.rst
+++ b/docs/manpages/ndnsec.rst
@@ -1,9 +1,6 @@
 ndnsec
 ======
 
-:program:`ndnsec` is a command-line toolkit to perform various NDN security
-management operations.
-
 Synopsis
 --------
 
@@ -14,6 +11,9 @@
 Description
 -----------
 
+:program:`ndnsec` is a command-line toolkit to perform various NDN security
+management operations.
+
 The NDN security data are stored in two places: **Public Information Base**
 (PIB) and **Trusted Platform Module** (TPM). The :program:`ndnsec` toolkit
 provides a command-line interface for managing and using the NDN security data.
@@ -58,3 +58,12 @@
 
 :doc:`unlock-tpm <ndnsec-unlock-tpm>`
   Unlock the TPM.
+
+Exit Status
+-----------
+
+Generally, :program:`ndnsec` commands exit with status 0 if the requested
+operation was completed successfully. On error, a nonzero status is returned.
+Individual commands may use certain nonzero exit codes to indicate that a
+more specific error has occurred. Please consult the respective man pages
+for more information.