Fixed the documentation of release()

I'd started to edit it to remove implementation details and got
distracted, leaving it half edited.

src/ZODB/interfaces.py

@@ -1069,7 +1069,9 @@ class IMVCCStorage(IStorage):
         """
 
     def release():
-        """Release resources held by
+        """Release resources held by the storage instance.
+
+        The storage instance won't be used again after this call.
         """
 
     def poll_invalidations():