strict-bytestring wrapper is undocumented

[thomie]

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}