[T5] fix lm lables in docstring
Add better explanation to T5 `lm_labels` dosctring.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=h1) Report
> Merging [#3529](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/75ec6c9e3a7de6cc3e2920f3bb531e7c840b8ada&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3529/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3529      +/-   ##
==========================================
+ Coverage   77.80%   77.81%   +0.01%     
==========================================
  Files         100      100              
  Lines       17062    17062              
==========================================
+ Hits        13275    13277       +2     
+ Misses       3787     3785       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3529/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `81.52% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3529/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `94.98% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3529/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.94% <0.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3529/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.32% <0.00%> (+0.17%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=footer). Last update [75ec6c9...8692264](https://codecov.io/gh/huggingface/transformers/pull/3529?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
