missing HexString documentation

[Walter Bright]

On 2/7/2018 11:29 AM, Ralph Doncaster wrote:
> I just did a quick check, and with DMD v2.078.1, the hexString template 
> increases code size by ~300 bytes vs the hex literal. So yet one more reason to 
> prefer the hex literals.

Indeed it does, and that is the result of a poor implementation of hexString. 
I've figured out how to fix that, and hope to make a PR for it shortly.

   https://issues.dlang.org/show_bug.cgi?id=18397