Clarify that `length_params.max_length` is not a hard constraint
# What does this PR do ?

Update docstring describing `length_params.max_length` to say that generated sequences may actually exceed this length, since this can be surprising.

**Collection**: nlp

# Changelog 
- Clarify that `length_params.max_length` is not a hard constraint

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**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [X] Documentation


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# Additional Information

I need someone to confirm whether the same doc change should be applied to this `asr` file, that seems to be copied from `nlp`:
https://github.com/NVIDIA/NeMo/blob/82e1f4f9f1a280edb70088bf30bf0a099152e185/nemo/collections/asr/modules/transformer/text_generation.py#L79

@stevehuang52 would you be able to confirm by any chance?