[Issue 18120] Document setting DRT options via the environment

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18120

--- Comment #6 from Steven Schveighoffer <schveiguy at yahoo.com> ---
I think that's the wrong place to document. They should be documented elsewhere
(but the document should be linked), because the options are scattered all over
the place (GC, cycle detection, unittest, etc.).

One document to show how to USE the DRT options should be a nice top-level
article.

--