[codex] clarify UDF signature annotations

[therealclvn]

Closes #757.

What changed

• Clarified in the UDF signatures section that the function name and Python type annotations are part of the UDF entrypoint signature.
• Added a short unannotated vs annotated apply_datacube example so users can translate the rendered Sphinx signature back to Python code.
• Added a changelog entry for the documentation fix.

Validation

• git diff --check
• Parsed the added Python code examples with ast.parse.

Note: I did not run a full Sphinx docs build because Sphinx is not installed in the available local Python environments.