updating docs - adding few tests to tokenizers

docs/source/main_classes/tokenizer.rst

@@ -1,6 +1,14 @@
 Tokenizer
 ----------------------------------------------------
 
+The base class ``PreTrainedTokenizer`` implements the common methods for loading/saving a tokenizer either from a local file or directory, or from a pretrained tokenizer provided by the library (downloaded from HuggingFace's AWS S3 repository).
+
+``PreTrainedTokenizer`` is the main entry point into tokenizers as it also implements the main methods for using all the tokenizers:
+
+- tokenizing, converting tokens to ids and back and encoding/decoding,
+- adding new tokens to the vocabulary in a way that is independant of the underlying structure (BPE, SentencePiece...),
+- managing special tokens (adding them, assigning them to roles, making sure they are not split during tokenization)
+
 ``PreTrainedTokenizer``
 ~~~~~~~~~~~~~~~~~~~~~~~~