- What is it? - What does it do? - Why does it do that?

This could all easily fit in the top-level comments of a main() function or the help text of a CLI app.

- What is the API?

This could be gleaned from the code, either by reading it or by generating automatic documentation from it.

- What does it return?

This is commonly documented in function code.

- What are some examples of proper, real world usage (that don't involve foo/bar but instead, real world inputs/outputs I'd likely see)?

This is typically in comments or help text if it's a CLI app.