Update RFC-00xx-special-functions.md

Author: 0x00b1

RFC-00xx-special-functions.md

@@ -1918,6 +1918,8 @@ Spherical Hankel function of the first kind:
 
 $$h_{n}^{1}(z)=\frac{\sqrt{\frac{\pi}{2}}H_{n +\frac{1}{2}}^{1}(z)}{\sqrt{z}},$$
 
+where $H_{n}^{1}$ is the Hankel function of the first kind.
+
 ##### Parameters
 
 **n** ([Tensor](https://pytorch.org/docs/stable/tensors.html#torch.Tensor)) – equation order. If $z$ is a number, $n$ must be a tensor.