Summary
Preserve module-level docstrings for type aliases documented with next-line triple-quoted strings. Previously, aliases to class/type objects (e.g., int) were documented only as “alias of …”, ignoring the alias docstrings.

Related Issue: #101

Reproduction steps

1. Minimal project (extensions = ['sphinx.ext.autodoc']) with module:
   • SimpleInt = int followed by a next-line docstring
   • MyInt: TypeAlias = int followed by a next-line docstring
   • IntListOrUnion: TypeAlias = Union[List[int], int] followed by a next-line docstring
2. Build with .. automodule:: using :members: and :undoc-members:.

Observed behavior (before fix)

• SimpleInt: only “alias of int” (alias docstring ignored)
• MyInt: only “alias of int” (alias docstring ignored)
• IntListOrUnion: shows alias docstring and “alias of Union[List[int], int]”
• No stack trace; this is a rendering/content inconsistency.

Root cause

• Autodoc chooses ClassDocumenter for aliases to class/type objects and sets doc_as_attr, but it does not merge module-level alias docstrings from ModuleAnalyzer. Aliases to typing expressions use DataDocumenter, which preserves module-level docstrings via attr_docs.

Fix

• Bias selection toward DataDocumenter for module-level aliases when a module-level alias docstring exists or a TypeAlias annotation is present and the alias points to a type/class with a different __name__. This preserves alias docstrings.
• Defensive: when ClassDocumenter still handles such aliases under doc_as_attr, merge the module-level alias docstring retrieved from ModuleAnalyzer before appending “alias of …”.

Tests

• Added tests/roots/test-ext-autodoc/target/type_alias_docstrings.py with aliases covering annotated, unannotated, complex typing expressions, and docstring-free cases.
• Added assertions in tests/test_ext_autodoc_automodule.py::test_type_alias_docstrings verifying that all alias docstrings are rendered and “alias of …” lines are included.

Local test environment notes

• Used Python 3.10 in a virtual environment for local testing.
• Local tests passed (subset focused on autodoc tests).

Expected outcome

• Docstrings for aliases to builtin types, typing expressions, and user-defined classes are rendered alongside “alias of …”.
• No regressions to existing autodoc behavior.