Documentation of toTitle needs update

[glchapman]

It appears the change proposed in issue 167 got committed so that title-casing only takes place after spaces. However, the documentation still reads: "The first letter of the input is converted to title case, as is every subsequent letter that immediately follows a non-letter".

[Bodigrim]

I think the easiest thing to do is to quote in haddocks a reference (but slow) impementation of toTitle: https://github.com/haskell/text/blob/02ee6771165e883eda516d61eb96c8156f6b7bb8/tests/Tests/Properties/Text.hs#L142-L148