[DGD] Innacuracy with DGD read_file documentation.

[Steve Wooster]

I was having trouble getting whitelisted, so I figured I'd send this to
the mailing list instead of directly to Dworkin...

I think I've found a minor inaccuracy with the documentation for
read_file. It says:

ERRORS
    If the offset is out of range or the returned string would be too
    large, an error will result.

However, from my own tests, it looks like it returns nil when the offset
is out of range, instead of raising an error. I believe I'm using
version 1.2.86 of DGD.

-Steve Wooster