Clarify resolver heuristics with inline documentation

[Erotemic]

Summary

• document the grabdata cache regression test expectations to highlight why the stub fails
• annotate the editable-resolver logic to explain alias normalization and finder path handling
• clarify the relative symlink normalization rationale within ub.symlink

Testing

• pytest tests/test_download.py::test_grabdata_missing_content_length tests/test_links.py::test_symlink_preserves_relative_target tests/test_import.py::test_modname_to_modpath_editable_suffix -q

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Codex Task

[Erotemic]

Split into https://github.com/Erotemic/ubelt/pull/170 https://github.com/Erotemic/ubelt/pull/171 https://github.com/Erotemic/ubelt/pull/172