[iostream.forward.overview] Typedef-names are mentioned in a weird way

[iostream.forward.overview]/7 currently says
> Other *typedef-name*s define instances of class templates specialized for `char` or `wchar_t` types.

It's arguably wrong to say so because these typedef-names don't cause anything to be defined. Also, it seems better to say "specializations" instead of "instances" here (see also #3903). Perhaps we should either rephrase or delete this paragraph.