Update man_keymgr.rst about generated TSIG key format

doc/man_keymgr.rst

@@ -32,7 +32,10 @@ Basic options
 
 **-t**, **--tsig** *tsig_name* [*tsig_algorithm*] [*tsig_bits*]
   Generates a TSIG key. TSIG algorithm can be specified by string (default: hmac-sha256),
-  bit length of the key by number (default: optimal length given by algorithm).
+  bit length of the key by number (default: optimal length given by algorithm). The generated 
+  TSIG key is only displayed on `stdout`: the command does not create a file, nor include the
+  key in a keystore.
+  
 
 Config options
 ..............