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 
@sgugger noting: this PR is ready (tests have passed)<|||||>Thanks a lot!