[PretrainedTokenizer] add some of the most important functions to the doc (#27313)

docs/source/en/main_classes/tokenizer.md

@@ -55,6 +55,8 @@ to a given token).
 
 [[autodoc]] PreTrainedTokenizer
     - __call__
+    - add_tokens
+    - add_special_tokens
     - apply_chat_template
     - batch_decode
     - decode
@@ -69,6 +71,8 @@ loaded very simply into 🤗 transformers. Take a look at the [Using tokenizers
 
 [[autodoc]] PreTrainedTokenizerFast
     - __call__
+    - add_tokens
+    - add_special_tokens
     - apply_chat_template
     - batch_decode
     - decode