[PATCH, libstdc++] Doc changes for constexpr additions for C++20 status.

[Jonathan Wakely]

On 05/09/19 15:45 -0400, Ed Smith-Rowland via libstdc++ wrote:
>Here is a patch to the libstdc++ docs re constexpr additions. They 
>reflect the latest macro assignments AFAICT.
>
>Constexpr interator reqs are implemented in 9.1, the rest for 10.1.
>
>Ok?
>
>Should I bother adding the Constexpr interator requirements to the 
>gcc-9 branch docs?

Yes please, it would be good to have that part in the gcc-9 docs. No
need to regen the HTML on the branch at this time though.

Thanks!