Correct the documentation to specify that has_back_reference<> must be a

metafunction rather than a traits class.
Correct typos in the example.


[SVN r23730]

libs/python

@@ -1 +1 @@
-Subproject commit 27653b7fbf09200677582664c1a441249ac7c5b8
+Subproject commit 5e82d653a17ea971d71c1cbddc9d9a7080c61d86