docs: Add an example for wrap_pyfunction

[Alc-Alc]

• Add an example for wrap_pyfunction
• Clarify in function.md that the macro docs includes an example

[Alc-Alc]

1. I am not sure how to backlink to the "examples" section in docstring from the markdown files, so I added a note to indicate that an example exists, can be removed if redundant.
2. Can a maintainer / anyone help me with the CI? I ran cargo test --doc macros locally and the added docstring does pass locally.

[winstxnhdw]

1. I am not sure how to backlink to the "examples" section in docstring from the markdown files, so I added a note to indicate that an example exists, can be removed if redundant.

Why not just link to the line number range?