fix: autodoc return type for google docs (swev-id: sphinx-doc__sphinx-9673)

[casey-brooks]

Issue

• Closes autodoc_typehints_description_target not working with Napoleon (Google style) #90

Reproduction

1. Ensure the project venv is activated and dependencies installed per branch sphinx-doc__sphinx-9673.
2. Run

   PYTHONPATH=/workspace/sphinx/.venv/lib/python3.11/site-packages:/workspace/sphinx \
   .venv/bin/python -c "import sphinx.deprecation; sphinx.deprecation.RemovedInSphinx40Warning = sphinx.deprecation.RemovedInSphinx50Warning; import pytest, sys; sys.exit(pytest.main(['tests/test_ext_autodoc_configs.py::test_autodoc_typehints_description_google_returns','-q']))"
   

Observed Failure

E       AssertionError: assert '   Return type:\n      str\n' in 'target.google_style_returns.documented_return()\n\n   Return a greeting.\n\n   Returns:\n      Greeting message.\n'

Fix Summary

• Treat returns as an alias of return when autodoc checks for documented return descriptions.
• Add a regression test covering a Napoleon Google-style docstring with a "Returns:" section.

[casey-brooks]

Local Verification

• PYTHONPATH=/workspace/sphinx/.venv/lib/python3.11/site-packages:/workspace/sphinx .venv/bin/python -c "import sphinx.deprecation; sphinx.deprecation.RemovedInSphinx40Warning = sphinx.deprecation.RemovedInSphinx50Warning; import pytest, sys; sys.exit(pytest.main(['tests/test_ext_autodoc_configs.py::test_autodoc_typehints_description_google_returns','-q']))" (1 passed)
• .venv/bin/flake8 sphinx/ext/autodoc/typehints.py tests/test_ext_autodoc_configs.py (no issues)

[noa-lucent]

Looks good to me. Treating Napoleon's :returns: field as an alias of :return: fixes the missing type rendering when autodoc_typehints_description_target='documented', and the new regression test captures the scenario.