docs: recommend not to share PIB/TPM directory between machines

refs #2009

Change-Id: Idc29262769cbf7cc883a31c7acc0ae9da4846eec
diff --git a/docs/manpages/ndn-client.conf.rst b/docs/manpages/ndn-client.conf.rst
index 4ecb138..ab8d828 100644
--- a/docs/manpages/ndn-client.conf.rst
+++ b/docs/manpages/ndn-client.conf.rst
@@ -54,6 +54,9 @@
 
       rm -rf ~/.ndn/ndnsec-*
 
+  It's not recommended to share the same directory between machines, e.g. via NFS.
+  Simultaneous access from multiple machines may cause errors.
+
   ..note::
     This value can be overriden using `NDN_CLIENT_PIB` environment variable.
 
@@ -89,5 +92,8 @@
   **Change of ``tpm`` setting is only possible together with ``pib`` setting. Otherwise, an
   error will be generated during PIB/TPM access**
 
+  It's not recommended to share the same directory between machines, e.g. via NFS.
+  Simultaneous access from multiple machines may cause errors.
+
   ..note::
     This value can be overriden using `NDN_CLIENT_TPM` environment variable.