[Doc] 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

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Additional details

See also: https://github.com/uxlfoundation/oneDNN/issues/3995