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

[Joel Brobecker]

> > 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.

One of the things that struck me when I read the diff is that
I asked myself why this conversion is necessary. The patch is
only a small piece of the documentation, so perhaps the reasons
are clear already.  But otherwise, it might be worth explaining.

-- 
Joel