Document GHC-81303

Here's minimal repro steps:
```haskell
{-# LANGUAGE OrPatterns #-}

foo :: Either Int Int -> String
foo (Left _; Right _) = "boom"
```
This code is all fine, bu changing either `_` to `x` triggers the error:
```
An or-pattern may not bind term or type variables such as ‘x’ [GHC-81303]
```

Search the error-message index for GHC-81303 and use the minimal OrPatterns reproduction in the issue as the starting example. Document why replacing either wildcard with x triggers the error, and consider the work complete when the index explains the restriction clearly for this example.

Written by the indexing model from the issue text.