missing HexString documentation

[Walter Bright]

On 2/7/2018 4:08 PM, Mike Franklin wrote:
> Don't search for "[betterC]".  Instead, use "betterC" (without the brackets).
> 
> https://issues.dlang.org/buglist.cgi?quicksearch=betterc&list_id=219390
> 
> We can't reliably rely on informal conventions.

I used the wrong URL. This is the right one (a keyword search, not a text search):

https://issues.dlang.org/buglist.cgi?keywords=betterC&list_id=219394&resolution=---

which lists 13 issues. Two of them were missing, and I annotated them with the 
keyword betterC, so it's 15.