Adds a note to resize the token embedding matrix when adding special … (#11120)

* Adds a note to resize the token embedding matrix when adding special tokens

* Remove superfluous space

src/transformers/tokenization_utils_base.py

@@ -825,7 +825,13 @@ class SpecialTokensMixin:
         special tokens are NOT in the vocabulary, they are added to it (indexed starting from the last index of the
         current vocabulary).
 
-        Using : obj:`add_special_tokens` will ensure your special tokens can be used in several ways:
+        .. Note::
+            When adding new tokens to the vocabulary, you should make sure to also resize the token embedding matrix of
+            the model so that its embedding matrix matches the tokenizer.
+
+            In order to do that, please use the :meth:`~transformers.PreTrainedModel.resize_token_embeddings` method.
+
+        Using :obj:`add_special_tokens` will ensure your special tokens can be used in several ways:
 
         - Special tokens are carefully handled by the tokenizer (they are never split).
         - You can easily refer to special tokens using tokenizer class attributes like :obj:`tokenizer.cls_token`. This