Note missing constexpr for random number generator classes.

[Ed Smith-Rowland]

Benjamin Kosnik wrote:
>> Well, constexpr is missing from the C++ front-end, as a C++0x core
>> feature. I'm not sure it's worth pointing out everywhere in the
>> library docs that we can't be close to the current specifications in
>> this area.
>>     
>
> I think this might be useful when the constexpr work is completed, since
> many of the constexpr uses have some implementation bits done. This
> would provide a checklist for the fixups. I'm not quite sure it's
> many more places than what is already noted. So, I would be inclined to
> specify this in detail.
>   
I did a quick search through the standard and the random constexpr doc 
patch plus what is already in the docs should just about do it.  I did 
the random patch to be consistent with the rest of the docs really.

I'll do a walk through to make sure.
> The random status in general is just a half stab, probably there are
> more comments that you two could provide but I wanted to just have a
> better idea of the C++0x status. I'm in favor of less brevity if it
> helps us all get on the same page WRT what the status is and what is
> needed.
>
>   
>> We should also do the same as regards concepts, for
>> example, etc., etc, ... If it's not there already, probably we need
>> just a blanket disclaimer.
>>     
>
> Well, the concepts bits are detailed in many places. I think finer
> detail is helpful.
>
> -benjamin
>
>