Update quicktour docs to showcase the use of truncation
# What does this PR do?
Currently, running the tokenizer batch example on https://huggingface.co/transformers/quicktour.html gives an error 

```
Asking to truncate to max_length but no maximum length is provided and the model has no predefined maximum length. Default to no truncation.
```

This PR fixes the above by passing the `max_length` param explicitly (instead of depending on it having a default, which might not be the case for all models).

The fix also adds clarity to the statement in the docs above this example
> If your goal is to send them through your model as a batch, you probably want to pad them all to the same length, truncate them to the maximum length the model can accept and get tensors back

## 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 
