Document the type used by `FIT_STATIC_FUNCTION` and `FIT_STATIC_LAMBDA_FUNCTION`

The type of variable created by `FIT_STATIC_FUNCTION` or `FIT_STATIC_LAMBDA_FUNCTION` could be different. Perhaps a template alias such as `static_function` or `static_lambda_function` can be used by the user to know the exact type of the declared variable.