Finished implementing gc.get_referrents(): dealt with error and end
cases, wrote docs, added a test.
diff --git a/Doc/lib/libgc.tex b/Doc/lib/libgc.tex
index 310c582..eea8d8c 100644
--- a/Doc/lib/libgc.tex
+++ b/Doc/lib/libgc.tex
@@ -99,6 +99,19 @@
\versionadded{2.2}
\end{funcdesc}
+\begin{funcdesc}{get_referrents}{*objs}
+Return a list of objects directly referred to by any of the arguments.
+The referrents returned are those objects visited by the arguments'
+C-level \cfunction{tp_traverse} methods (if any), and may not be all
+objects actually directly reachable. \cfunction{tp_traverse} methods
+are supported only by objects that support garbage collection, and are
+only required to visit objects that may be involved in a cycle. So,
+for example, if an integer is directly reachable from an argument, that
+integer object may or may not appear in the result list.
+
+\versionadded{2.3}
+\end{funcdesc}
+
The following variable is provided for read-only access (you can
mutate its value but should not rebind it):