quantum mechanics - What is the Wigner function of $|nranglelangle m|$?

I have been searching in the literature for the Wigner function of $|n \rangle \langle m|$. For $n=m$ it can be found in page 120 of Barnett and Radmore's Methods in Theoretical Quantum Optics and it is given by $$ W_n(\alpha) = \frac{2}{\pi} (-1)^n \exp(-2 |\alpha|^2) L_n(4 |\alpha|^2), $$ where $L_n$ is a Laguerre polynomial.

The result I am looking for is used in the QuTiP software package (specifically, in the wigner.py file) to calculate Wigner functions given the matrix representation of state in the Fock basis.

From the code (see line 223 for example) it seen that the Wigner function for $|n \rangle \langle m|$ is related to a generalized Laguerre polynomial, they reference the book Measuring the Quantum State of Light (Cambridge University Press, 1997) by Ulf Leonhardt, but I checked the book and it does not contain the expression that I am looking for.

Thanks in advance for any help/references you can provide.