[wwwdocs] Document addition of <barrier> C++ header in C++11

[Jonathan Wakely]

On 14/01/21 10:42 +0000, Jonathan Wakely wrote:
>Pushed to wwwdocs.
>

>commit 768ecbe7af00a219cfdff9fba96874a0fe9c94bb
>Author: Jonathan Wakely <jwakely@redhat.com>
>Date:   Thu Jan 14 10:41:43 2021 +0000
>
>    Document addition of <barrier> C++ header in C++11

Doh, that was meant to say GCC 11 not C++11.