Documented gc.get_referrers() as dangerous.

SF bug 793822

Doc/lib/libgc.tex

@@ -96,6 +96,11 @@ be listed among the resulting referrers.  To get only currently live
 objects, call \function{collect()} before calling
 \function{get_referrers()}.
 
+Care must be taken when using objects returned by
+\function{get_referrers()} because some of them could still be under
+construction and hence in a temporarily invalid state. Avoid using
+\function{get_referrers()} for any purpose other than debugging.
+
 \versionadded{2.2}
 \end{funcdesc}