strict-bytestring wrapper is undocumented

197317b03cf9ab82def73b1c3b1fa4925738e812 added the `strict-bytestring` wrapper, but the User Guide doesn't mention it.

In most wrappers, `AlexInput` is a tuple, whereas in `strict-bytestring` (and also in `basic-bytestring`, since #51) it is a record:

```
data AlexInput = AlexInput { alexChar :: {-# UNPACK #-} !Char,
alexStr :: !ByteString.ByteString,
alexBytePos :: {-# UNPACK #-} !Int64}
```