Documentation mismatch in Preprocessing data
## Environment info
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- `transformers` version: 4.10.0.dev0
- Platform: macOS-10.16-x86_64-i386-64bit
- Python version: 3.9.2
- PyTorch version (GPU?): 1.9.0 (False)
- Tensorflow version (GPU?): 2.6.0 (False)
- Flax version (CPU?/GPU?/TPU?): 0.3.4 (cpu)
- Jax version: 0.2.19
- JaxLib version: 0.1.70
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help

@sgugger @SaulLu 
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- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
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## Information

There seems a conflict in [ Utilities for tokenizers ](https://huggingface.co/transformers/internal/tokenization_utils.html?highlight=truncation#transformers.tokenization_utils_base.PreTrainedTokenizerBase.__call__) and [Preprocessing data](https://huggingface.co/transformers/preprocessing.html?highlight=truncation#everything-you-always-wanted-to-know-about-padding-and-truncation).
In **Preprocessing data**, For **`truncation_strategy = True`**, It states "truncate to a maximum length specified by the max_length argument or the maximum length accepted by the model if no max_length is provided (max_length=None). This will only truncate the first sentence of a pair if a pair of sequences (or a batch of pairs of sequences) is provided."
whereas for the same in **Utilities for tokenizers**, it states "Truncate to a maximum length specified with the argument max_length or to the maximum acceptable input length for the model if that argument is not provided. This will truncate token by token, removing a token from the longest sequence in the pair if a pair of sequences (or a batch of pairs) is provided.".


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## Expected behavior

In Preprocessing_data documentation , `truncation_strategy=True` must match with `longest_first` instead of `only_first`.

<!-- A clear and concise description of what you would expect to happen. -->

Indeed. Would you mind opening a PR with the change?