GHC-94210 "Pattern match has inaccessible right hand side" is undocumented

For this example code -

```haskell
{-# LANGUAGE GADTs #-}

data HList a where
HNil :: HList ()
HCons :: head -> HList tail -> HList (head, tail)

exampleHList :: HList (String, (Int, (Bool, ())))
exampleHList = HCons "Tom" (HCons 25 (HCons True HNil))

hListHead :: HList (a,b) -> a
hListHead HNil = undefined
hListHead (HCons head tail) = head
```

`ghc 9.0.2` provides the following undocumented warning -

```
example.hs:11:1: warning: [-Woverlapping-patterns]
Pattern match has inaccessible right hand side
In an equation for ‘hListHead’: hListHead HNil = ...
|
11 | hListHead HNil = undefined
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
```