Document new overloads of `wiener_lpdf`

#### Summary:

Added by @Franzi2114 in https://github.com/stan-dev/math/pull/2822

They have a write up from last summer: https://link.springer.com/article/10.3758/s13428-023-02179-1

The new signatures are 
```
wiener_lpdf(real, real, real, real, real, real) => real
wiener_lpdf(real, real, real, real, real, real, real, real) => real
```


#### Current Version:
v2.35.0
