Use __init__ type hints in "description" mode

Previously, `__init__` type hints were not used when documenting a class
using `autodoc_typehints="description"`. This was done to prevent
documentation for parameters from showing up twice, both for the class
and the `__init__` special method. As the new ``autodoc_typehint_undoc``
option provides a better way to prevent this bad behavior by placing the
user in control of where the type hints are added, it is now safe to add
type hints for documented `__init__` parameters.

Closes #8178

sphinx/ext/autodoc/typehints.py

@@ -42,8 +42,6 @@ def merge_typehints(app: Sphinx, domain: str, objtype: str, contentnode: Element
         return
     if app.config.autodoc_typehints != 'description':
         return
-    if objtype == 'class' and app.config.autoclass_content not in ('init', 'both'):
-        return
 
     try:
         signature = cast(addnodes.desc_signature, contentnode.parent[0])