Update documentation on absolute position embed seq2seq models
Update documentation on seq2seq models with absolute positional embeddings to be in line with BERT and GPT2. 
Issue #19581. For models with absolute positional embeddings, it is not usually a good idea to left-pad as if positional embeddings are not shifted the right amount for each element in the batch the results will not be correct.

Further work may be required to incorporate `position_ids` kwargs (but possibly for both encoder/decoder) for these models similar to BERT and GPT2. However, at the very least the documentation should be updated to be consistent with BERT/GPT2 to provide a warning.
_The documentation is not available anymore as the PR was closed or merged._