Improve documentation of pooler_output in ModelOutput
# What does this PR do?
Improves the doc string for pooler_output in modeling_outputs.py – making it more clear, and opening its availability to a more generic use-case than just BERT-family of models.

**Motivation**:
I was writing a `cls_pooler` for a sentence embeddings usage, and initially thought this is the CLS token output from the last layer – which is not the case, that would just be `last_hidden_state[0]`

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?
@sgugger 
Thanks, you just need to run `make style` on your branch to fix the styling issue.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>> Thanks, you just need to run `make style` on your branch to fix the styling issue.

Oh I missed this message @sgugger – just noticed. 
Created a fresh PR – https://github.com/huggingface/transformers/pull/13228