add bug number and explanation to changes/bug2385; call it a feature

changes/bug2385

@@ -1,5 +1,9 @@
- o Minor bugfixes:
-   - Clear keys and key-derived material left on the stack in rendservice.c
-     and rendclient.c
+ o Minor features (security):
+   - Clear keys and key-derived material left on the stack in
+     rendservice.c and rendclient.c. This should make us more
+     forward-secure against cold-boot attacks and the like. Fix for
+     bug 2385.
+
    - Check return value of crypto_pk_write_private_key_to_string() in
-     end_service_load_keys()
+     end_service_load_keys(). This should make us more forward-secure
+     against cold-boot attacks and the like. Fix for bug 2385.