[3.13] gh-130117: Document why nested `Union`, `Literal`, and `Annotated` types referenced through a type alias are not flattened (GH-130119) (#133488)

gh-130117: Document why nested `Union`, `Literal`, and `Annotated` types referenced through a type alias are not flattened (GH-130119)

(cherry picked from commit b936ccdb6f6bd11250b4e638b6fa2c239907ca58)

Co-authored-by: Valentin Berlier <berlier.v@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>