io FEATURE automatically add action elem as needed

Refs cesnet/netopeer2#1045
diff --git a/src/messages_client.h b/src/messages_client.h
index 2286753..dda44dc 100644
--- a/src/messages_client.h
+++ b/src/messages_client.h
@@ -174,6 +174,8 @@
  * Note that created object can be sent via any NETCONF session that shares the context
  * of the @p data.
  *
+ * @note In case of action, the \<action\> element is added automatically and should not be in @p data.
+ *
  * @param[in] data NETCONF RPC data as a data tree.
  * @param[in] paramtype How to further manage data parameters.
  * @return Created RPC object to send via a NETCONF session or NULL in case of (memory allocation) error.
@@ -185,6 +187,8 @@
  *
  * For details, see ::nc_rpc.
  *
+ * @note In case of action, the \<action\> element is added automatically and should not be in @p xml_str.
+ *
  * @param[in] xml_str NETCONF RPC data as an XML string.
  * @param[in] paramtype How to further manage data parameters.
  * @return Created RPC object to send via a NETCONF session or NULL in case of (memory allocation) error.