missing HexString documentation

[Adam D. Ruppe]

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).

By contrast, the word "octal" only occurs 100 times through all 
of Phobos, and the octal template is used only 15 times, 
excluding its own unit tests.