-The read operation was not successful, probably because no data was
-available. Call SSL_get_error() with the return value B<ret> to find out,
-whether an error occurred.
+The read operation was not successful. The reason may either be a clean
+shutdown due to a "close notify" alert sent by the peer (in which case
+the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set
+(see L<SSL_shutdown(3)|SSL_shutdown(3)>,
+L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>). It is also possible, that
+the peer simply shut down the underlying transport and the shutdown is
+incomplete. Call SSL_get_error() with the return value B<ret> to find out,
+whether an error occurred or the connection was shut down cleanly
+(SSL_ERROR_ZERO_RETURN).
+
+SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
+only be detected, whether the underlying connection was closed. It cannot
+be checked, whether the closure was initiated by the peer or by something
+else.