- explain why ring.h is marked as a public header

- fix typo in extension module name

src/persistent/SETUP.cfg

@@ -2,9 +2,16 @@
 
 # Mark an "exported" header for use from other packages.
 # This is not needed for headers only used within the package.
+#
 header  cPersistence.h
+
+
+# This is included by cPersistence.h, so all users of cPersistence.h
+# have to be able to include this indirectly.
+#
 header  ring.h
 
+
 <extension cPersistence>
   source     cPersistence.c
   source     ring.c
@@ -13,7 +20,7 @@ header  ring.h
   depends-on ring.h
 </extension>
 
-<extension cPersistence>
+<extension cPickleCache>
   source     cPickleCache.c
   source     ring.c