Update documentation on "flags" parameter in dnnl_lstm_backward_primitive_desc_create()

[lacak-sk]

Summary

There is flags argument in dnnl_lstm_backward_primitive_desc_create() which is documented as Unused.
But there should be noted that for backward primitive is used: RNN flags (@ref dnnl_rnn_flags_t).

URLs

Include pointers to documents that are impacted.

Additional details

See also: #3995