missing HexString documentation

[Adam D. Ruppe]

On Thursday, 8 February 2018 at 10:52:35 UTC, Kagamin wrote:
> Or have a function specifically for unix permissions, like
> int unix(int r, int w, int x, int gr, int gw, int gx, int ur, 
> int uw, int ux);
> It might be even more readable.

I actually personally prefer binary: 0b_1_111_101_000 which 
visually corresponds with ls's output: drwxr-xr-x.

But octal is the way they are usually done in C. The comments in 
druntime are because that had to be translated from the common 
convention.