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

This was added to the `add_tokens` method, but was forgotten on the `add_special_tokens` method.

See the updated docs: https://191874-155220641-gh.circle-artifacts.com/0/docs/_build/html/internal/tokenization_utils.html?highlight=add_special_tokens#transformers.tokenization_utils_base.SpecialTokensMixin.add_special_tokens

closes https://github.com/huggingface/transformers/issues/11102