Add position_ids in TFElectra models docstring
Just a small thing, but it was forgotten when adding those I guess.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5021?src=pr&el=h1) Report
> Merging [#5021](https://codecov.io/gh/huggingface/transformers/pull/5021?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/1affde2f10c653e36601dd7a3e6a2525ae7ced57&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/5021/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5021?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5021   +/-   ##
=======================================
  Coverage   77.26%   77.27%           
=======================================
  Files         128      128           
  Lines       21847    21847           
=======================================
+ Hits        16880    16882    +2     
+ Misses       4967     4965    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5021?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5021/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `91.28% <ø> (ø)` | |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/5021/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `76.35% <0.00%> (-2.30%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5021/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.20% <0.00%> (-0.24%)` | :arrow_down: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5021/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `34.07% <0.00%> (+5.41%)` | :arrow_up: |

------

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