Remove anachronistic docs about G++ template instantiation

[Gerald Pfeifer]

On Thu, 8 Mar 2012, Jonathan Wakely wrote:
> The manual claims a future version of G++ will support a hybrid
> instantiation model, which I don't think is still planned, and
> describes extern templates as an extension when they are in C++11.
> 
>         * doc/extend.texi (Template Instantiation): Remove anachronisms.

I was waiting for a C++ frontend maintainer to chime in.  The
patch per se looks good to me, and based on some others mails
around template instantiations I think you can go ahead.

Gerald