[Generation] fix docs for decoder_input_ids
Fix docs for decoder input ids.
Pinging @sshleifer, @sgugger and @LysandreJik for notification.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=h1) Report
> Merging [#5306](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/135791e8ef12802ceb21a4abbb3a93f7da1bf390&el=desc) will **decrease** coverage by `0.84%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5306/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5306      +/-   ##
==========================================
- Coverage   79.30%   78.45%   -0.85%     
==========================================
  Files         138      138              
  Lines       24283    24283              
==========================================
- Hits        19258    19052     -206     
- Misses       5025     5231     +206     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5306/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.77% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5306/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.26% <ø> (+0.12%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5306/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.78% <0.00%> (-74.20%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=footer). Last update [135791e...d4e9c3f](https://codecov.io/gh/huggingface/transformers/pull/5306?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
