channels: Warn against executing multiple commands in single channel

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/channels.c

@@ -2783,6 +2783,12 @@ error:
        }
    }
 @endcode
+ *
+ * @warning In a single channel, only ONE command can be executed!
+ * If you want to executed multiple commands, allocate separate channels for
+ * them or consider opening interactive shell.
+ * Attempting to run multiple consecutive commands in one channel will fail.
+ * See RFC 4254 Section 6.5.
  *
  * @see ssh_channel_request_shell()
  */