[doc/tracepoint] Add a note for assigning values of inferior to trace state variables

[Eli Zaretskii]

> From: Hui Zhu <teawater@gmail.com>
> Date: Fri, 16 Sep 2011 16:56:36 +0800
> Cc: Eli Zaretskii <eliz@gnu.org>
> 
> I talk about a bug about assigning values of inferior to trace state variables
> http://sources.redhat.com/ml/gdb/2011-07/msg00009.html

That thread died with Tom asking for details.  Was the issue resolved
off-list?

> this issue can be handle by convert value to (uint64_t), for example:
> teval $tmp=(uint64_t)a

Isn't this specific to the platform where this runs?  I don't think we
should have platform-specific information, and such minor details at
that, in general sections.