missing HexString documentation

[Walter Bright]

On 2/7/2018 6:39 PM, Adam D. Ruppe wrote:
> On Thursday, 8 February 2018 at 01:55:19 UTC, Walter Bright wrote:
>> No, because their usage by druntime is nearly nonexistent.
> 
> Only because they're not supported!
> 
> Code like `0xsomething // octal something else` is found a whopping 200 times in 
> druntime (granted btw all in the core.sys bindings).

Nearly all of that is in 3 files, and most are copy/pasta of the same groups 
lines for different systems.

I didn't find any uses of x"string" at all, or my grep-fu is wanting.