improve doctstring for tf and pt generate() method
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3327?src=pr&el=h1) Report
> Merging [#3327](https://codecov.io/gh/huggingface/transformers/pull/3327?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e8f44af5bf44a79f102678f5d7bb737cd6da3b52&el=desc) will **decrease** coverage by `0.10%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3327/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3327?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3327      +/-   ##
==========================================
- Coverage   77.10%   77.00%   -0.11%     
==========================================
  Files         100      100              
  Lines       16953    16953              
==========================================
- Hits        13071    13054      -17     
- Misses       3882     3899      +17     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3327?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `84.43% <ø> (-3.23%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3327/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.09% <ø> (+0.13%)` | :arrow_up: |

------

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