Clarify `use_fast` parameter in docstring (#20840)

* clarify use_fast parameter

* make style

* remove check frameworks, apply review

src/transformers/models/auto/tokenization_auto.py

@@ -524,7 +524,9 @@ class AutoTokenizer:
                 In case the relevant files are located inside a subfolder of the model repo on huggingface.co (e.g. for
                 facebook/rag-token-base), specify it here.
             use_fast (`bool`, *optional*, defaults to `True`):
-                Whether or not to try to load the fast version of the tokenizer.
+                Use a [fast Rust-based tokenizer](https://huggingface.co/docs/tokenizers/index) if it is supported for
+                a given model. If a fast tokenizer is not available for a given model, a normal Python-based tokenizer
+                is returned instead.
             tokenizer_type (`str`, *optional*):
                 Tokenizer type to be loaded.
             trust_remote_code (`bool`, *optional*, defaults to `False`):