[Generation] fix docs for decoder_input_ids (#5306)
* fix docs
* Update src/transformers/modeling_utils.py
* Update src/transformers/modeling_tf_utils.py
* Update src/transformers/modeling_tf_utils.py
* Update src/transformers/modeling_utils.py
* Update src/transformers/modeling_tf_utils.py
* Update src/transformers/modeling_utils.py