Document that get_referrers can return unreachable but uncollected objects.

Fixes #505453.

Doc/lib/libgc.tex

@@ -83,6 +83,13 @@ Return the list of objects that directly refer to any of objs. This
 function will only locate those containers which support garbage
 collection; extension types which do refer to other objects but do not
 support garbage collection will not be found.
+
+Note that objects which have already been dereferenced, but which live
+in cycles and have not yet been collected by the garbage collector can
+be listed among the resulting referrers.  To get only currently live
+objects, call \function{collect()} before calling
+\function{get_referrers()}.
+
 \versionadded{2.2}
 \end{funcdesc}
 

Misc/ACKS

@@ -472,6 +472,7 @@ Edward Welbourne
 Rickard Westman
 Truida Wiedijk
 Gerry Wiener
+Bryce "Zooko" Wilcox-O'Hearn
 Sue Williams
 Frank Willison
 Greg V. Wilson