[RFA] Document breakage of Solaris Thread Debugging

[Joel Brobecker]

> 2006-06-20  Joel Brobecker  <brobecker@adacore.com>
> 
>         * PROBLEMS: Document threads/2137.

Oops, I forgot to mention that I already applied this on the branch.
I thought that we had already done this, but I noticed during the
pre-release routine work that this wasn't the case. So I documented
it the best I could in order to avoid having to wait and then hold
the release for another day.

-- 
Joel