[RFA/NEWS] document addition of set/show/unset substitute-path

[Eli Zaretskii]

> Date: Tue, 8 Aug 2006 15:41:19 -0700
> From: Joel Brobecker <brobecker@adacore.com>
> 
> What do you guys think of the following patch. I tried to keep
> things relatively short and succint. Does it need to be expanded
> a bit or is it clear enough?

I think it's clear enough.  Please commit this.

Thanks.