[examples] improve block_size warning message
there is an odd warning inside the examples wrt model_max_length value e.g. in `run_clm.py`
```
01/28/2023 16:03:50 - WARNING - __main__ - The tokenizer picked seems to have a very large `model_max_length` 
(1000000000000000019884624838656). Picking 1024 instead. You can change that default value by passing --block_size xxx.
```

As the models now can work with much longer sequence lengths (bloom, opt, others) should it try to truncate it to 1024?

But actually for me what stood out is `1000000000000000019884624838656` - when I see such huge numbers it usually means a bug, so it is worrying and suggests that either I am doing something wrong or there is a bug somewhere.

so this PR is proposing to reword the message to be as informative, but not as scary:

```
01/28/2023 16:03:50 - WARNING - __main__ - The chosen tokenizer supports a `model_max_length` that is longer than the 
default `block_size` value of 1024. If you would like to use a longer `block_size` up to `tokenizer.model_max_length` you 
can override this default with `--block_size xxx`.
```
_The documentation is not available anymore as the PR was closed or merged._