Remove MBart subclass of XLMRoberta in tokenzier docs (#16546)

* Remove MBart subclass of XLMRoberta in tokenzier

* Fix style

* Copy docs from MBart50 tokenizer

src/transformers/models/mbart/tokenization_mbart_fast.py

@@ -62,9 +62,8 @@ class MBartTokenizerFast(PreTrainedTokenizerFast):
     Construct a "fast" MBART tokenizer (backed by HuggingFace's *tokenizers* library). Based on
     [BPE](https://huggingface.co/docs/tokenizers/python/latest/components.html?highlight=BPE#models).
 
-    [`MBartTokenizerFast`] is a subclass of [`XLMRobertaTokenizerFast`]. Refer to superclass
-    [`XLMRobertaTokenizerFast`] for usage examples and documentation concerning the initialization parameters and other
-    methods.
+    This tokenizer inherits from [`PreTrainedTokenizerFast`] which contains most of the main methods. Users should
+    refer to this superclass for more information regarding those methods.
 
     The tokenization method is `<tokens> <eos> <language code>` for source language documents, and ``<language code>
     <tokens> <eos>``` for target language documents.