[T5] Add training documenation 
- Fixes T5 docstring regarding pretraining
- Add detailed description on how to process input and target for T5 training
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=h1) Report
> Merging [#3507](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/601ac5b1dc1438f00d09696588f2deb0f045ae3b&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3507/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3507   +/-   ##
=======================================
  Coverage   77.79%   77.80%           
=======================================
  Files         100      100           
  Lines       17051    17051           
=======================================
+ Hits        13265    13266    +1     
+ Misses       3786     3785    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3507/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `97.58% <ø> (ø)` | |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3507/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `81.29% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3507/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `94.98% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3507/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.94% <0.00%> (+0.13%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=footer). Last update [601ac5b...ea7e6a8](https://codecov.io/gh/huggingface/transformers/pull/3507?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
