Document that get_referrers can return unreachable but uncollected objects.
Fixes #505453.
diff --git a/Doc/lib/libgc.tex b/Doc/lib/libgc.tex
index b332fe2..67ac64f 100644
--- a/Doc/lib/libgc.tex
+++ b/Doc/lib/libgc.tex
@@ -83,6 +83,13 @@
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}