Clean RAG docs and template docs
Author: sgugger
Pull request: yes

Followup from #7345, this cleans up the documentation for RAG (since it was merged while I was working) and update the templates to the new docstrings.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=h1) Report
> Merging [#7348](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/129fdae04033fe4adfe013b734deaec6ec34ae2e?el=desc) will **decrease** coverage by `0.92%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7348/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7348      +/-   ##
==========================================
- Coverage   76.68%   75.75%   -0.93%     
==========================================
  Files         181      181              
  Lines       34851    34853       +2     
==========================================
- Hits        26724    26402     -322     
- Misses       8127     8451     +324     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JhZy5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yYWcucHk=) | `76.98% <ø> (ø)` | |
| [src/transformers/retrieval\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9yZXRyaWV2YWxfcmFnLnB5) | `91.27% <ø> (ø)` | |
| [src/transformers/tokenization\_rag.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcmFnLnB5) | `71.11% <100.00%> (+1.34%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mdW5uZWwucHk=) | `18.53% <0.00%> (-75.51%)` | :arrow_down: |
| [src/transformers/tokenization\_fsmt.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZnNtdC5weQ==) | `20.34% <0.00%> (-74.90%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `25.32% <0.00%> (-73.63%)` | :arrow_down: |
| [src/transformers/modeling\_layoutlm.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sYXlvdXRsbS5weQ==) | `25.06% <0.00%> (-69.40%)` | :arrow_down: |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `42.10% <0.00%> (-57.90%)` | :arrow_down: |
| [src/transformers/activations\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9uc190Zi5weQ==) | `54.16% <0.00%> (-20.84%)` | :arrow_down: |
| ... and [14 more](https://codecov.io/gh/huggingface/transformers/pull/7348/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=footer). Last update [129fdae...930dd4b](https://codecov.io/gh/huggingface/transformers/pull/7348?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).