Finished implementing gc.get_referrents(): dealt with error and end

cases, wrote docs, added a test.

Doc/lib/libgc.tex

@@ -99,6 +99,19 @@ objects, call \function{collect()} before calling
 \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):