Warn that settrace() is implementation rather than language
definition, per Guido's request in off-list email.
diff --git a/Doc/lib/libsys.tex b/Doc/lib/libsys.tex
index 3b25e5a..2cac03e 100644
--- a/Doc/lib/libsys.tex
+++ b/Doc/lib/libsys.tex
@@ -475,7 +475,11 @@
the Python debugger.\index{debugger} The function is
thread-specific; for a debugger to support multiple threads, it must
be registered using \function{settrace()} for each thread being
- debugged.
+ debugged. \note{The \function{settrace()} function is intended only
+ for implementing debuggers, profilers, coverage tools and the like.
+ Its behavior is part of the implementation platform, rather than
+ part of the language definition, and thus may not be available in
+ all Python implementations.}
\end{funcdesc}
\begin{funcdesc}{settscdump}{on_flag}