missing HexString documentation

[Adam D. Ruppe]

On Wednesday, 7 February 2018 at 19:38:35 UTC, Ali Çehreli wrote:
> It sounds so natural. I forgot; what was the argument against 
> it?

0o was denied basically just because we felt it wasn't necessary 
to have in the language at all; that it was rare enough and the 
library *can* do it, so the library *should* do it.

And at the time, I totally agreed! And in some cases, I still do 
- I think D programmers ought to know the technique so they can 
use it for their own niches.

Just in the years since, we see `0x40; // octal 0100` instead of 
`octal!100` since the cost of the library import is higher than 
the cost of converting by hand to hex or binary, which are still 
built into the language.