[wwwdocs] Mention libstdc++-v3 <regex> in 4.9 changes.html

[Jonathan Wakely]

On 2 October 2013 11:41, Paolo Carlini wrote:
>
> Minimally, I would talk about "improved support": the evolution from
> -std=c++0x to -std=c++11 meant that we aren't in experimental mode anymore.

>From speaking to Jason he's pretty adamant it's still experimental for now :-)

My understanding was that -std=c++11 changed just because the standard
was published, not because our support was finished.

I agree we don't need to say experimental on the release notes page though.