Fix examples titles and optimization doc page
Author: sgugger
Pull request: yes

This PR addresses two things:
- first, some of the titles were a bit messy in the navigation bar in the examples and optimization page, fixed that
- second, it expands the optimization documentation, adding mentions of which classes/functions go with which backend (since there is no TF prefix) and expand existing docstrings or add them if missing.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=h1) Report
> Merging [#5408](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/87716a6d072b2b66415ce43086c73b04e63fe0fe&el=desc) will **increase** coverage by `0.21%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5408/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5408      +/-   ##
==========================================
+ Coverage   77.69%   77.90%   +0.21%     
==========================================
  Files         140      140              
  Lines       24334    24336       +2     
==========================================
+ Hits        18906    18960      +54     
+ Misses       5428     5376      -52     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb24ucHk=) | `96.05% <100.00%> (+0.05%)` | :arrow_up: |
| [src/transformers/optimization\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb25fdGYucHk=) | `57.65% <100.00%> (+0.38%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.78% <0.00%> (-74.20%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `73.37% <0.00%> (-25.00%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `79.72% <0.00%> (-1.38%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.18% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `90.22% <0.00%> (+0.31%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `79.59% <0.00%> (+0.33%)` | :arrow_up: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `83.84% <0.00%> (+1.53%)` | :arrow_up: |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `94.20% <0.00%> (+2.17%)` | :arrow_up: |
| ... and [6 more](https://codecov.io/gh/huggingface/transformers/pull/5408/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=footer). Last update [87716a6...b839c40](https://codecov.io/gh/huggingface/transformers/pull/5408?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).