update roberta docstring as well

pytorch_transformers/tokenization_roberta.py

@@ -64,7 +64,11 @@ PRETRAINED_POSITIONAL_EMBEDDINGS_SIZES = {
 
 class RobertaTokenizer(GPT2Tokenizer):
     """
-    RoBERTa BPE tokenizer, derived from the GPT-2 tokenizer. Peculiarities: Byte-level BPE
+    RoBERTa BPE tokenizer, derived from the GPT-2 tokenizer. Peculiarities:
+        - Byte-level Byte-Pair-Encoding
+        - Requires a space to start the input string => will add a space is there isn't.
+          As a consequence, this tokenizer `encode` and `decode` method will not conserve
+          the absence of a space at the beginning of a string: `tokenizer.decode(tokenizer.encode("Hello")) = " Hello"
     """
     vocab_files_names = VOCAB_FILES_NAMES
     pretrained_vocab_files_map = PRETRAINED_VOCAB_FILES_MAP