Close #6518: autodoc: Add autodoc_type_aliases

autodoc_type_aliases allows to keep user defined type alises not
evaluated in the generated document.

doc/usage/extensions/autodoc.rst

@@ -515,6 +515,44 @@ There are also config values that you can set:
 
       New option ``'description'`` is added.
 
+.. confval:: autodoc_type_aliases
+
+   A dictionary for users defined `type aliases`__ that maps a type name to the
+   full-qualified object name.  It is used to keep type aliases not evaluated in
+   the document.  Defaults to empty (``{}``).
+
+   The type aliases are only available if your program enables `Postponed
+   Evaluation of Annotations (PEP 563)`__ feature via ``from __future__ import
+   annotations``.
+
+   For example, there is code using a type alias::
+
+     from __future__ import annotations
+
+     AliasType = Union[List[Dict[Tuple[int, str], Set[int]]], Tuple[str, List[str]]]
+
+     def f() -> AliasType:
+         ...
+
+   If ``autodoc_type_aliases`` is not set, autodoc will generate internal mark-up
+   from this code as following::
+
+     .. py:function:: f() -> Union[List[Dict[Tuple[int, str], Set[int]]], Tuple[str, List[str]]]
+
+        ...
+
+   If you set ``autodoc_type_aliases`` as
+   ``{'AliasType': 'your.module.TypeAlias'}``, it generates a following document
+   internally::
+
+     .. py:function:: f() -> your.module.AliasType:
+
+        ...
+
+   .. __: https://www.python.org/dev/peps/pep-0563/
+   .. __: https://mypy.readthedocs.io/en/latest/kinds_of_types.html#type-aliases
+   .. versionadded:: 3.3
+
 .. confval:: autodoc_warningiserror
 
    This value controls the behavior of :option:`sphinx-build -W` during