Doc tokenizer (#6110)

* Start doc tokenizers

* Tokenizer documentation

* Start doc tokenizers

* Tokenizer documentation

* Formatting after rebase

* Formatting after merge

* Update docs/source/main_classes/tokenizer.rst

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Address comment

* Update src/transformers/tokenization_utils_base.py

Co-authored-by: Thomas Wolf <thomwolf@users.noreply.github.com>

* Address Thom's comments

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Thomas Wolf <thomwolf@users.noreply.github.com>

docs/source/internal/tokenization_utils.rst

@@ -0,0 +1,38 @@
+Utilities for Tokenizers
+------------------------
+
+This page lists all the utility functions used by the tokenizers, mainly the class
+:class:`~transformers.tokenization_utils_base.PreTrainedTokenizerBase` that implements the common methods between
+:class:`~transformers.PreTrainedTokenizer` and :class:`~transformers.PreTrainedTokenizerFast` and the mixin
+:class:`~transformers.tokenization_utils_base.SpecialTokensMixin`.
+
+Most of those are only useful if you are studying the code of the tokenizers in the library.
+
+``PreTrainedTokenizerBase``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.tokenization_utils_base.PreTrainedTokenizerBase
+    :special-members: __call__
+    :members:
+
+
+``SpecialTokensMixin``
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.tokenization_utils_base.SpecialTokensMixin
+    :members:
+
+
+Enums and namedtuples
+~~~~~~~~~~~~~~~~~~~~~
+.. autoclass:: transformers.tokenization_utils_base.ExplicitEnum
+
+.. autoclass:: transformers.tokenization_utils_base.PaddingStrategy
+
+.. autoclass:: transformers.tokenization_utils_base.TensorType
+
+.. autoclass:: transformers.tokenization_utils_base.TruncationStrategy
+
+.. autoclass:: transformers.tokenization_utils_base.CharSpan
+
+.. autoclass:: transformers.tokenization_utils_base.TokenSpan