[Tokenizer doc] Clarification about `add_prefix_space` (#24368)

* nits

* more details

* fixup

* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

---------
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

src/transformers/generation/configuration_utils.py

@@ -143,8 +143,10 @@ class GenerationConfig(PushToHubMixin):
             If set to int > 0, all ngrams of that size can only occur once.
         bad_words_ids(`List[List[int]]`, *optional*):
             List of token ids that are not allowed to be generated. In order to get the token ids of the words that
-            should not appear in the generated text, use `tokenizer(bad_words, add_prefix_space=True,
-            add_special_tokens=False).input_ids`.
+            should not appear in the generated text, make sure to set `add_prefix_space=True` when initializing the
+            tokenizer, and use `tokenizer(bad_words, add_special_tokens=False).input_ids`. The `add_prefix_space`
+            argument is only supported for some slow tokenizers, as fast tokenizers' prefixing behaviours come from
+            `pre tokenizers`. Read more [here](https://huggingface.co/docs/tokenizers/api/pre-tokenizers).
         force_words_ids(`List[List[int]]` or `List[List[List[int]]]`, *optional*):
             List of token ids that must be generated. If given a `List[List[int]]`, this is treated as a simple list of
             words that must be included, the opposite to `bad_words_ids`. If given `List[List[List[int]]]`, this

src/transformers/generation/logits_process.py

@@ -546,8 +546,10 @@ class NoBadWordsLogitsProcessor(LogitsProcessor):
     Args:
         bad_words_ids (`List[List[int]]`):
             List of list of token ids that are not allowed to be generated. In order to get the token ids of the words
-            that should not appear in the generated text, use `tokenizer(bad_words, add_prefix_space=True,
-            add_special_tokens=False).input_ids`.
+            that should not appear in the generated text, make sure to set `add_prefix_space=True` when initializing
+            the tokenizer, and use `tokenizer(bad_words, add_special_tokens=False).input_ids`. The `add_prefix_space`
+            argument is only supported for some slow tokenizers, as fast tokenizers' prefixing behaviours come from
+            `pre tokenizers`. Read more [here](https://huggingface.co/docs/tokenizers/api/pre-tokenizers).
         eos_token_id (`Union[int, List[int]]`):
             The id of the *end-of-sequence* token. Optionally, use a list to set multiple *end-of-sequence* tokens.
     """

src/transformers/generation/tf_logits_process.py

@@ -292,7 +292,10 @@ class TFNoBadWordsLogitsProcessor(TFLogitsProcessor):
     Args:
         bad_words_ids (`List[List[int]]`):
             List of list of token ids that are not allowed to be generated. In order to get the tokens of the words
-            that should not appear in the generated text, use `tokenizer(bad_word, add_prefix_space=True).input_ids`.
+            that should not appear in the generated text, make sure to set `add_prefix_space=True` when initializing
+            the tokenizer, and use `tokenizer(bad_words, add_special_tokens=False).input_ids`. The `add_prefix_space`
+            argument is only supported for some slow tokenizers, as fast tokenizers' prefixing behaviours come from
+            `pre tokenizers`. Read more [here](https://huggingface.co/docs/tokenizers/api/pre-tokenizers).
         eos_token_id (`int`):
             The id of the *end-of-sequence* token.
     """